ERBB2 and breast carcinoma: For example, palbociclib IC50 for the commonly used HR+ /HER2- breast cancer line MCF7 was 48.3 μM in4 or 13.3 μM in5, despite numerous studies showing that 1 μM palbociclib or lower causes an efficient long-term G1 arrest in MCF7 cells9–16.