Progesterone receptors (PR, comprising A and B isoforms) are upregulated in response to ER signalling in normal and malignant breast tissues.3 Antibodies used to detect PR for clinical and investigative purposes largely detect both PR isoforms and using these, PR has been established as a biomarker of good prognosis in breast cancer.4 Higher levels of PR expression are associated with a good response to tamoxifen,4,5 which until the development of aromatase inhibitors (AIs) was the major first-line adjuvant endocrine therapy for all cases of ER-positive (ER+) breast cancer. Here, CYP19A1 is linked to breast cancer.