37 CFR 2.91 - Declaration of interference.
(a) An interference will not be declared between two applications or between an application and a registration except upon petition to the Director. Interferences will be declared by the Director only upon a showing of extraordinary circumstances which would result in a party being unduly prejudiced without an interference. In ordinary circumstances, the availability of an opposition or cancellation proceeding to the party will be deemed to remove any undue prejudice.
(b) Registrations and applications to register on the Supplemental Register, registrations under the Act of 1920, and registrations of marks the right to use of which has become incontestable are not subject to interference.