Strategies to reduce cdk4 activity in transformed cells. Downregulation of cyclin D1: Compounds that downregulate cyclin D1 levels include antiestrogens and retinoic acid. Rapamycin decreases the stability of both cyclin D1 mRNA and protein. Ras pathways have been linked to both the synthesis of cyclin D1 and its assembly with cdk4, events that may contribute to the mechanism of action of agents affecting these pathways. The cdk inhibitor flavopiridol also transcriptionally represses cyclin D1 expression. Destabilization of cdk4: Prior to its assembly with cyclin D, newly synthesized cdk4 first associates with a cytoplasmic chaperone complex containing cdc37/Hsp 90 to ensure its stabilization and proper folding (44). Geldanamycin binds and inactivates Hsp 90, causing the degradation of its substrates, including newly synthesized cdk4. Reactivation of p16^INK4A: Demethylating agents, such as 5-aza 2’doxycytidine, as well as histone deaceytlase inhibitors, can cause re-expression of p16^INK4A in cell lines in which the gene is transcriptionally silenced by methylation. Short p16^INK4A-derived peptides can induce G1 arrest when introduced into cells expressing wild-type Rb and may serve as a model for small molecule design.