4.Tumor must be HER2-positive and RB-proficient. RB (Retinoblastoma protein)-proficiency is determined by tumor biopsy demonstrating RB normal and p16in4a low by immunohistochemistry. RB proficiency means that there is an intact RB pathway indicative of responsiveness to PD-0332991. RB staining is scored on an absent (no nuclear staining), weak (nuclear staining less than observed in endothelial cells and stromal cells surrounding the tumor), positive (nuclear staining at or above surrounding tissue) (0, 0.5, 1 respectively). P16ink4a is a routine clinical stain that is scored using absent, weak, positive, strong (0,1,2,3 respectively). Tumors will be scored using [p16]/[RB], where a score of less than 3 is required for inclusion. RB loss is expected to occur in less than 15% of cases.