CDK4 and triple-negative breast carcinoma: Studies on MDA-MB-231, a triple negative breast cancer cell line, showed that CDK4 is degraded more efficiently and PROTAC containing palbociclib (22a) is more potent (DC50 CDK4 = 12.9 nM, DC50 CDK6 = 34.1 nM) than 22b (DC50 CDK4 = 97 nM, DC50 CDK6 = 300 nM).