KLLN is a p53 target gene with DNA binding function and represents a highly plausible candidate breast cancer predisposition gene. We screened for predisposing variants in 860 high-risk breast cancer families using high resolution melt analysis. A germline c.339_340delAG variant predicted to cause premature termination of the protein after 57 alternative amino acid residues was identified in 3/860 families who tested negative for BRCA1 and BRCA2 mutations and in 1/84 sporadic breast cancer cases. However, the variant was also detected in 2/182 families with known BRCA1 or BRCA2 mutations and in 2/464 non-cancer controls. Furthermore, loss of the mutant allele was detected in 2/2 breast tumors. Our data suggest that pathogenic mutations in KLLN are rare in breast cancer families and the c.339_340delAG variant does not represent a high-penetrance breast cancer risk allele.