We found that CHEK2 is the only cell cycle checkpoint kinase gene that is more likely to be mutated in the germ line than somatically (threefold enrichment in ER+/HER2− breast cancer relative to ATM and >50-fold enrichment relative to ATR, P = 3.7 × 10−12) (Fig. 2A). Here, CHEK2 is linked to breast carcinoma.