The concept of so-called “poisonous divisionals” and “toxic priorities” has been troubling professionals and patent applicants/owners for quite some time now and many questions are still unanswered. These terms refer to cases where a European patent application of the same patent family is held against another European application or patent as prior art under Article 54 (3) EPC. In T 557/13 the competent Technical Board of Appeal has now referred questions of law to the Enlarged Board of Appeal of the European Patent Office, to hopefully shed some light on the matter and bring clarity to patent owners and applicants. Intricately connected with the question of whether an application of the same patent family may constitute prior art under Article 54 (3) EPC is the question of whether a patent claim may enjoy partial priorities. For this reason, the first set of referred questions is aimed to clarify whether a claim of a European patent application may enjoy partial priorities even though alternatives are not explicitly listed in the claim, and if so, under which conditions. Although the Enlarged Board had touched on the topic of partial priorities earlier in decision G 2/98, divergent case law has developed since then on its interpretation, warranting a new referral to the Enlarged Board. In case partial priority is denied, the Enlarged Board is also asked whether parent or divisional applications of a European patent application may be cited as prior art under Article 54 (3) EPC against each other at all. A decision on the matter may be expected in a year or two.