In a recent study of patients treated with palbociclib who had received prior chemotherapy, 100 mg palbociclib was associated with lower rates of grade 3/4 neutropenia than 125 mg; further, correlation of response by dose suggested that use of biomarkers such as pRB and Ki67 could indicate the use of palbociclib treatment and reduce toxicity [20]. This evidence concerns the gene RB1 and neutropenia.