Intriguingly in this context is that several previous studies have reported associations between SNPs in intron 4 of ESR1 and oestrogen-dependent traits,[35–39] including breast cancer risk.[36] The most significant OXTR SNP in the present study, rs237902 is located in exon 3 and has previously been associated with behavioural phenotypes,[40, 41] as well as susceptibility to preterm birth.[42] However, the functional relevance of this SNP remains to be clarified. Here, OXTR is linked to breast cancer.