PGR and breast cancer: Firstly, it has been suggested that considerable heterogeneity exists in the etiology of breast cancer.23 Breast cancer can be classified according to estrogen receptor and/or progesterone receptor expression, with each type having different clinical, pathological and molecular features.24 The positive association between breast cancer and smoking may be diluted or masked in studies that treat breast cancer as a single outcome, as we did in our analyses.