CYP19A1 and breast cancer: A previous Early Breast Cancer Trialists' Collaborative Group (EBCTCG) meta-analysis of trials of aromatase inhibitors versus tamoxifen in postmenopausal women with early-stage oestrogen receptor (ER)-positive breast cancer has shown that aromatase inhibitors reduce recurrence rates by about 30% compared with tamoxifen over the 5-year treatment period.