ESR1 and breast cancer: Notably, breast cancers with impactful mutations in C2 domain (21 cases, 2.1%), helical domain (105 cases, 10.7%), or PI4K domain (138 cases, 14.1%) were more likely to be ER- and PR-positive (P = 0.032, P < 0.001, P < 0.001, for ER; P = 0.030, P < 0.001, P < 0.001, for PR; respectively).