Estrogen receptor-positive (ER+) breast cancer accounts for 67-80% of all breast cancer subtypes. Endocrine therapies are initially given as part of first-line treatment, however 20% of the patients develop resistance. The overactivation of cyclin dependent kinase 4/6 (CDK4/6) is one of the underlying mechanisms. Hence the addition of CDK4/6 inhibitors such as palbociclib in combination with en…