The biosynthesis of active steroids (e.g. oestradiol [E2] and androstenediol [Adiol]) in cancer tissues mainly depends on the following three enzymatic pathways: aromatase (AROM), 17β-hydroxysteroid dehydrogenase (17β-HSD), and steroid sulphatase (STS) (Scheme 1).6 The gene discussed is CYP19A1; the disease is cancer.