CYP3A4 and breast cancer: Similarly, a reduced-function CYP3A422 variant has been linked to higher plasma levels of several drugs (e.g., the breast cancer therapy exemestane) [21], suggesting that CYP3A4 polymorphism may also heighten exposure (and toxicity risk) for TKIs (Tirosine Kinase Inhibitors) predominantly cleared by CYP3A4.