ESR1 and breast cancer: The luminal/estrogen receptor-alpha positive (ER+) subtypes are the most commonly diagnosed breast cancers (60–80%), with patients being classified as either good outcome Luminal A, or worse outcome Luminal B. Patients with Luminal A tumors have good overall survival, in part, because these tumors are slow growing, typically respond to endocrine therapy, and have infrequent TP53 mutations [5], [6].