Updated project metadata. Estrogen receptor positive (ER+) breast cancer is the most common type of breast cancer. Despite the great efficacy of endocrine therapies, resistance remains a problem. Therefore, there is an immediate need for new, effective therapies in ER+ BC. In this study, we have explored the role of a potent CDK4/6 inhibitor called palbociclib. In order to identify alterations in protein abundance between the responsive and resistant setting, we generated a panel of palbociclib-sensitive and resistant cell lines and did quantitative, shotgun proteomics using dimethyl labelling. Palbociclib sensitive cell lines (wt-MCF7 or MCF7-LTED) were cultured in phenol red-free RPMI supplemented with 10% FBS and 1nM estradiol or 10% dextran-coated charcoal (DCC) respectively. Thereafter, palbociclib resistant cell lines were generated using 1μΜ palbociclib concentration. Samples were harvested at baseline and at the point of resistance.