Intriguingly, palbociclib could suppress ER+Ki67+ (PNP), but not PR+Ki67+ (PPP/NPP) cells, providing the potential explanation of why palbociclib has been only effective for advanced breast cancers and highlighting the importance of its combination with endocrine therapy to target both PNP and PPP/NNP populations. The gene discussed is PGR; the disease is breast cancer.