Two HR+/HER2+ cell lines were also included to search for strong common mechanisms of resistance that might occur in HR+ BC despite HER2 status, given that palbociclib has also shown efficacy in HER2+ BC models.19,24–26 We obtained reliable models of acquired resistance to palbociclib, as revealed by functional analyses investigating proliferation, IC50 and β-galactosidase expression, for all cell lines except CAMA1. This evidence concerns the gene ERBB2 and breast cancer.