ESR1 and breast cancer: Compared to older women, early-onset breast cancer was found to have tumor grade 3 (29% in YAW vs 17% in OW), estrogen receptor negativity (45% in YAW vs 23% in OW), triple-negative (32% in YAW vs 10% in OW), and higher proliferation index Ki-67 (25% in YAW vs 10% in OW) [28].