Isolated nucleic acid sequences encoding mammalian MDM2 binding protein and polypeptide sequences for the mammalian MDM2 binding protein are provided. Also provided are vectors containing these nucleic acid sequences, host cells which express these proteins and antibodies targeted to these proteins. In addition, methods and compositions for modulating the G.sub.1 phase of the cell cycle via altering expression and/or activity of a mammalian MDM2 binding protein are provided.