Cyclin E, along with the three cyclin D proteins and cyclin C, has been shown to represent a putative G1 cyclin on the basis of its cyclic pattern of mRNA expression, with maximal levels being detected near the G1/S boundary. cyclin E has been found to be associated with the transcription factor E2F in a temporally regulated manner. Cyclin E2 is a cyclin E-related protein that specifically interacts with Cdk2 and Cdk3 and with p27 and p21. Cyclin E2 expression peaks at the G1/S phase transition of the cell cycle, in parallel with cyclin E. Whereas cyclin E1 is expressed in most proliferating normal and tumor cells, cyclin E2 levels are low or undetectable in nontransformed cells, and are elevated in tumor-derived cells.