Since there is a phenotypic overlap of HBOC and LS, timely identification of individuals at-risk of a particular syndrome is crucial in order to optimize cancer risk management.<h4>Case presentation</h4>We report a novel pathogenic MSH2 mutation, c.2656G > T, which was identified in a 67-year-old female patient with breast cancer, who had previously tested negative for a deleterious mutation in the breast cancer susceptibility genes BRCA1, BRCA2, CHEK2 or RAD51C. The gene discussed is CHEK2; the disease is Leigh syndrome.