PGR and breast cancer: The most recent meta-analysis of all prospective studies, including three previously unpublished cohorts concluded that night shift work has little or no effect on breast cancer incidence,15 but this has been challenged.29–31 It has been suggested that recent exposure,29–32 initiating night work at young ages16,30,31 or before first pregnancy,33,34 and risk among pre-menopausal women for oestrogen receptor (ER) positive, progesterone receptor (PR) positive, or human epidermal growth factor-receptor 2 (HER2) positive tumours14,16,34,35 may be relevant, but the evidence is inconclusive.