RB1 and cancer: A peptide bearing the 84–103 (DAAREGFLATLVVHRAGAR) sequence of p16INK4a that interacts with CDK4 and CDK6 was found to inhibit phosphorylation of RB1 by CDK4/cyclin D1 in vitro, and block cell cycle progression when fused to Penetratin or TAT, in several cancer cell lines but not in an RB1-negative cell line [374,377] (Figure 6A,B).