Transcription regulator specifically required for multiciliate cell differentiation. Acts in a multiprotein complex containing E2F4 and E2F5 that binds and activates genes required for centriole biogenesis. Required for the deuterosome-mediated acentriolar pathway. Plays a role in mitotic cell cycle progression by promoting cell cycle exit. Modulates GMNN activity by reducing its affinity for CDT1. Heterodimer (via coiled-coil domain) with GMNN (via coiled-coil domain); targets GMNN to the nucleus. Can form homodimers (in vitro, via coiled-coil domain), but these are much less stable than the heterodimer formed with GMNN. Excluded from the nucleolus. Belongs to the geminin family.