Subsequent population-based studies have shown that a recurrent loss of function CHEK2 mutant, CHEK21100delC is present in approximately 1% of the population and in 5% of familial breast cancer families lacking a BRCA1 or BRCA2 mutation (Meijers-Heijboer et al, 2002; Fernandez-Novoa et al, 2004). Here, BRCA1 is linked to breast cancer.