PGR and breast cancer: This would be in contrast to the findings from Key et al. who reported no evidence of an association between premenopausal progesterone levels and breast cancer risk overall and no heterogeneity in estimates stratified by PR status.3 However, the number of cases of PR + (N = 158) and PR− (N = 61) breast cancer was small, and this analysis may have lacked power to detect modest associations in subgroups of cancers.