ESR1 and breast cancer: On an individual gene level, we observed >5-fold enrichment for mutations in CHEK2 (29 of 3382 versus 0 of 640, P = 0.001 after adjustment for multiple comparisons) in ER+/HER2− breast cancer samples relative to TNBCs, but no statistically significant enrichment for ATM, ATR, or CHEK1 mutations in either subtype (Fig. 1B).