Novel Mammalian Cyclins (CYL Genes) Expressed during G1

Polypeptide growth factors generally exert their proliferative effects on mammalian cells throughout the first gap phase (G1) of the cell cycle, during which they regulate the evolution of a genetic program that ultimately commits the cell to DNA synthesis (S phase) (Pardee 1989). The sequential steps that occur during the G1 interval are presumed to lead to the accumulation of labile regulatory proteins that irreversibly trigger S-phase entry. Once cells enter S phase, however, cell cycle control becomes relatively refractory to extracellular signals, and the processes governing DNA replication, preparation for cell division during a second gap phase (G2), and mitosis itself (M phase) rely more critically on intrinsic controls.