In patients with advanced ER+ breast cancer receiving daily oral doses of 125 mg palbociclib, the mean plasma concentration of palbociclib at steady-state is 116 ng/mL (259 nmol/L; ref. 26), indicating that CDK9 inhibitor synergy in palbociclib-resistant cells is achieved at, or near, clinically relevant concentrations of palbociclib, and could translate into using subtherapeutic combination dosing to avoid toxicities. This evidence concerns the gene CDK9 and breast carcinoma.