RB1 and cancer: Nevertheless, a potent and highly selective inhibitor of CDK4 and CDK6, called Palbociclib (PD-0332991), recently proved to be dependent on the presence of RB1 for its antiproliferative effect in several cancer cell lines and xenograft models [133] and a Phase I trial of PD-0332991 utilized RB1-deficiency as an exclusion criterion [119].