CHEK2 and in situ carcinoma: About a quarter of all ATM-related BCs and a third of CHEK2 BCs were in situ carcinomas and more than half of ATM and CHEK2-related BCs were diagnosed at stage I-II (59.1% and 55%, respectively), whereas 13.6% of the ATM BCs and 15% of the CHEK2 BCs were stage III-IV.