When diagnosed with breast cancer, their age at diagnosis is younger than those of patients with sporadic breast cancer.89 11, 16 The majority of breast tumours from CHEK2 c.1100delC heterozygotes are oestrogen receptor (ER) positive.417, 19 Furthermore, after the primary breast cancer diagnosis, carriers (both heterozygous and homozygous) more often develop contralateral breast cancer1620, 22 and tend to have worse survival than non-CHEK2 carriers.16, 1823 Previous studies were primarily conducted on retrospectively collected breast cancer cases, that were tested in a research setting. This evidence concerns the gene CHEK2 and breast cancer.