The first study to investigate effect modification by polymorphisms in circadian genes in the association between shift work and breast cancer risk was recently published by Monsees et al. [56] The study, which was conducted within the NHSII, observed that women homozygous for the minor allele (AA) of NPAS2 Ala394Thr with ≥24 months of shift work had a 2.83-times higher breast cancer risk compared to homozygous AA women with <24 months of shift work (95% CI 1.47-5.56). The gene discussed is NPAS2; the disease is breast cancer.