CHEK2 and breast cancer: We found that the mutation frequency of ERCC6 (1.8%) in breast tumors was similar to those of known breast cancer susceptibility genes, such as BRCA1 (2.9%), BRCA2 (2.9%), BLM (1.9%), FGFR2 (1.5%), and CHEK2 (1.0%) (Fig. 3a).