ESR1 and breast cancer: Aromatase,or CYP19A1, is a cytochrome P450 (CYP) enzyme with a crucial rolein estrogen biosynthesis.77 Aromatase haslong been a clinical target in the treatment of estrogen receptor-positive(ER+) breast cancer, with aromatase inhibitors (AIs) being a cornerstonein therapeutic approaches.78 Letrozoleis a prominent third-generation nonsteroidal AI,78 and understanding its CYP19A1 inhibition is crucial fordeveloping novel breast cancer treatments.