• Dr. Arwed Burrichter, Dr. Natalie Kirchhofer and Andreas Spinnrock

Deadly pitfalls and lessons learned - claiming priority at the EPO

In a recent high stakes CRISPR/Cas patent opposition case, the EPO Boards of Appeal have once again confirmed their strict and formalistic approach towards the (in)validity of priority claims. It is essential to know the EPO’s strict requirements for validly assigning the priority right.

Within 12 months of filing a first patent application, the applicant or its successor in title can file subsequent applications throughout the world, benefitting from the first application’s early filing date if priority is validly claimed. In a recent high stakes CRISPR/Cas patent opposition case, the EPO Boards of Appeal have once again confirmed their strict and formalistic approach towards the (in)validity of priority claims. It is essential to know the EPO’s strict requirements for validly assigning the priority right, particularly if the applicants are different between the priority and the subsequent application. Most importantly, the assignment must occur prior to filing the subsequent application. A late or missing assignment cannot be remedied retroactively. As in the recent CRISPR/Cas case, finding the priority claim to be invalid for formal reasons can lead to the invalidation of the entire patent if relevant prior art was filed or published in the priority year. This chapter explains the EPO’s requirements for a valid transfer of the priority right and summarises the relevant case law.


Priority right of the applicant or its successor in title

There are two essential requirements for validly claiming priority at the EPO: the ‘same invention’ and the ‘same applicant or successor in title’ requirement. This chapter concerns only the latter. The relevant provision, Article 87(1) of the European Patent Convention (EPC), stipulates: “Any person who has duly filed, in or for (a) any State party to the Paris Convention for the Protection of Industrial Property or (b) any Member of the World Trade Organization, an application for a patent, a utility model or a utility certificate, or his successor in title, shall enjoy, for the purpose of filing a European patent application in respect of the same invention, a right of priority during a period of twelve months from the date of filing of the first application.” The key questions for the ‘same applicant or successor in title’ criterion include:

  • Who is ‘any person’?
  • How can someone become their ‘successor in title’?

The priority right originates in the applicant of the first application. Therefore, in principle, the applicant of the first application and of the subsequent application must be the same, or the latter must be the ‘successor in title’ of the former. Since the priority right is an independent right, it can be transferred independently of the corresponding first application. Further, it may be transferred for only a part of the subject matter of the first application (partial priority, T 969/14) or for one or more countries only (T 62/05). There is no exhaustion of priority rights (ie, the same priority right might be validly claimed in more than one European patent application (T 15/01)). However, EPO case law is strict regarding the ‘same applicant or successor in title’ criterion. If the applicants of the priority filing differ from those of the later filing, the applicant of the later filing must have acquired the right for claiming priority of the earlier filing before the date of the later filing. Adding applicants for the later filing who were not yet named as applicants on the priority application is not a problem. However, if the priority application was filed by joint applicants, all applicants, or their successors in title, must be among the joint applicants of the later application for its priority claim to be valid. This is especially challenging for US applicants, where (at least before the America Invents Act), US priority applications needed to be filed in the names of the inventors, whereas the later international application was often filed in the name of the company. There are many cases, in which assignments from the inventors to the company are not performed properly or timely. It may also be dangerous to rely solely on employment contracts or prospective general assignments for proving an assignment of a particular priority right. Here, the devil is in the detail: it is not sufficient for the contract to stipulate only an obligation to assign (‘I agree to assign’ language). Instead, ‘I do hereby assign’ or other similar language should be included, indicating a present transfer of the inventor’s current and future patent rights to ensure that employee-created patent and priority rights are validly assigned to the employer. Whether such general prospective assignments can be used at all to transfer future priority rights is subject to the national law that the legal relationship between the priority and later applicants is governed by. A defective priority is easily challengeable by simply comparing the identities of the applicants on the priority and the later filing. In fact, according to the EPO, Article 87(1) of the EPC was purposely drafted to set out clear formal requirements that can be easily checked by patent offices and the public. For this reason, the right to claim priority was not tied to the (more complicated) question of inventorship, but to the (more formal) question of applicant status at the time of filing the priority and later application. Care should also be taken if a change in the applicant or inventor is recorded during the priority year. Unless such a change takes retroactive effect, which is rarely the case, or is accompanied by a valid transfer of rights, the ‘same applicant or successor in title’ criterion will continue to look to the original priority applicants. It is established EPO case law that, when assessing validity of a priority claim, the inventorship of the priority filing and the subsequent applications is irrelevant – it is the identity of the original applicant(s) that is decisive (T 5/05). If a transfer of the priority right occurred, this must have happened before the filing date of the subsequent application (T1201/14). A succession in title that occurred after the filing date of the subsequent application is insufficient (T 577/11). Retroactive transfer, even if permitted under national law (eg, US national law) is insufficient. If there are several applicants of the first application and one of them is the only applicant of the subsequent application, the other co-applicants must have transferred their priority rights (T 382/07). There is conflicting case law on whether assignments of priority rights need to be performed in writing in analogy to Article 72 of the EPC concerning the assignment of a European patent application (pro T 62/05; contra T 205/14). In general, regarding the validity of the assignments and necessary additional requirements, the EPO looks to the national law of the country in which the legal relationship between the previous and new rights holder is regulated, not the law of the country in which the first application was filed (T 1201/14). To be on the safe side, it is recommended that assignments of priority rights be made in writing and signed by all parties before the filing date of the later application. The EPO or opponents need only to challenge that no proper or timely assignment of the priority right occurred. The burden of proof for the valid transfer of the priority right is with the successor in title (ie, the applicant of the later filing). They must provide sufficient evidence of legal succession (T 1008/96). Copies of the transfer agreement may be sufficient, together with proof that the contents correspond to the original (affidavits) (T 493/06). Parties referring to applicable national law must provide appropriate evidence (T 1103/15). 


EPO decision

The grave consequences that a formally invalid priority claim can have, once again became clear in the first CRISPR/Cas9 case that reached the EPO’s Boards of Appeal (T 844/18). An exceptionally long four-day oral proceeding on the sole issue of priority took place from 13 to 16 January 2020 before life sciences Board of Appeal 3.3.08 (with Chairman Beat Stolz), which ruled in an enlarged five-member composition. It was clear that if the priority claim was found invalid, the claimed subject matter would lack novelty over the inventors’ own research publications that had come out in the priority year. The patent had been revoked in the first instance by the Opposition Division for exactly this reason. Nine parties had opposed the patent. The following fact pattern was at issue: the owners of the opposed patent were the Broad Institute, the Massachusetts Institute for Technology and Harvard College. The underlying Patent Cooperation Treaty (PCT) application claimed priority from 12 US provisional applications, which were filed in the name of different sets of inventors as applicants. One of the priority inventor-applicants had assigned its rights to the Rockefeller University. Neither that inventor nor the Rockefeller University were named as applicants on the subsequent PCT application. The patentee argued that this had been done intentionally, as the Rockefeller applicant had only contributed to particular subject matter that was not pursued in the PCT application. In fact, a different PCT application had been filed for the jointly invented subject matter naming Rockefeller University as co-applicant. The EPO was not convinced by these arguments and considered the priority claim to be invalid. The patentees appealed this decision based on three fundamental arguments:

  • Entitlement to priority should not be assessed by the EPO, but by competent national courts (in this case, US courts).
  • Based on proper linguistic analysis of ‘any person’ in Article 87 of the EPC, every applicant of a priority application would enjoy and could practice its own priority right individually. There was no requirement that all priority applicants needed to act jointly.
  • The respective national law should at least be applied when assessing who ‘any person’ could be. The applicant status is linked to the country of origin where the priority right arises. US law does not require all original applicants to be named on subsequent applications claiming priority.

The opponents responded that patentee’s position would be contrary to long-standing case law of the EPO and that there was no need for a change. The current practice would protect joint applicants from sharp practice. The EPO’s strict formal rules would have been well-known and could have easily been complied with. In view of the established case law, they asked, “should everyone else change or just the patentee?” All parties requested a referral to the Enlarged Board of Appeal should the board decide against them. In total, more than 300 documents were filed, including expert opinions by Professor Klaus-Jürgen Mellulis, former judge at the German Supreme Court; David Neuberger and Leonard Hoffman, former judges at the UK Supreme Court; Ursula Kinkeldey and Rudolf Teschemacher, former chairs of EPO Boards of Appeal; Tobias Bremi, judge at the Swiss Federal Patent Court; and David Kappos, former director of the USPTO, to name a few. In its preliminary opinion, the Board of Appeal summarised the arguments speaking for maintaining the present practice:

The practice is long standing. It may protect joint applicants against sharp practice by a sub-group of the joint applicants. The situation of the patentees in the present case could have been avoided if they had shown suffcient foresight. There is a risk of multiple subsequent applications. There could be other possibly negative consequences of changing this practice.

The Board of Appeal also summarised the arguments in favour of changing the present practice:

The joint applicant for the priority application who is not mentioned in the subsequent application would be in a better position: it would at least have the possibility of asserting its rights in any patent resulting from the priority application; the present practice means that loss of priority may be followed by loss of the patent. The appellants’ approach is more in line with the aim of the Paris Convention, that of facilitating international patenting as it relaxes the required formalities whilst safe-guarding those elements necessary for recognising a priority right under Article 87(1) EPC.

In the four-day oral hearing before the Board of Appeal, the pros and cons were discussed extensively. Many interested parties attended the public hearing. Whereas at the beginning of Day 3 the board had signalled an inclination to follow the patentee’s position and refer questions to the Enlarged Board of Appeal, it concluded the proceedings on Day 4 without any referral and confirming the first-instance finding of lack of priority and revocation of the patent. The established case law was again confirmed. Even with this decision, the dispute over the patentees’ patents on the CRISPR technology is not over. There are numerous parallel and divisional applications still pending in the same family as the opposed patent. Some list Rockefeller University as co-applicant and thus do not have the fatal priority defect. In addition, there are at least two other, unrelated appeal proceedings (EP 2 825 654 and EP 2 840 140) with similar priority issues that will be decided by different boards of appeal in the near future. Questions regarding the EPO’s ‘same applicant’ approach may be referred to the Enlarged Board of Appeal after all. Moreover, it seems that the patentees are preparing a petition for review.


Lessons learned and practical advice

In practice, the easiest way to circumvent any problems with the EPO’s ‘same applicant’ or ‘successor in title’ criterion is to file the subsequent application in the name of exactly the same applicants as the priority application. In the United States, parties should file both the US priority and the subsequent filings in the name of the company, not the inventors. Since the implementation of the America Invents Act, this is now possible. If there are multiple inventions with different sets of inventors, they should be separated into different priority applications from the outset. If the applicants of the priority application and the subsequent application are not identical, it should be ensured that all applicants of the priority application have transferred their priority right to at least one of the subsequent applicants before the filing date of the subsequent application. Otherwise the subsequent filing should at least formally list the priority applicants for which no assignment was concluded in time. Best practice is to perform priority assignments in writing, with both the assignor’s and assignee’s signature present. As an opponent, the ‘same applicant’ or ‘successor in title’ criterion should be checked before determining the cut-off date for prior art searches by simply checking the bibliographic information of the priority and subsequent filing for a mismatch in applicants. Since the burden of proof lies with the patentee, it makes sense strategically to contest the priority claim if relevant prior art was filed or published in the priority year.



Understanding the EPO’s strict case law is essential to avoid potentially deadly pitfalls in claiming priority. If the key aspects are understood and kept in mind already at the stage of filing any subsequent PCT or European Patent application, an unnecessary loss of European patents due to formal errors can be avoided. The future of the EPO’s approach towards priority is exciting, particularly in the field of life sciences, where much is at stake and the inventor’s own research results are regularly published in the priority year .


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This article first appeared in IAM Life Sciences: Key issues for senior life sciences executives 2020, a supplement to IAM, published by Globe Business Media Group - IP Division. To view the guide in full, please go to


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