CDK4 and breast cancer: Palbociclib (Ibrance, Pfizer, NY, NY, USA) is the first-in-class CDK4/6i receiving an accelerated approval from FDA in February 2015, in combination with letrozole as initial therapy for postmenopausal women HR-positive, HER2-negative advanced, or metastatic BC, based on the results from the phase II trial PALOMA-1/TRIO-18, which showed a significant benefit in terms of progression-free survival (PFS) for combination therapy with palbociclib and letrozole over letrozole alone (median PFS 20.2 vs. 10.2 months, hazard ratio (HR) 0.49, p = 0.0004) [47].