Progesterone metabolizing enzymes and pathways in MCF-7, MDA-MB-231, T-47D, and MCF-10A cells. Progesterone is converted to 5α-pregnane-3,20-dione (5α-P-3,20-dione) by 5αR, to 4-pregnen-3α-ol-20-one (3αHP) by 3α-HSO, or to 4-pregnen-20α-ol-3-one (20αDHP) by 20α-HSO. Each of the 4-pregnenes can be 5α-reduced by 5αR, in a one-way reaction, to the respective 5α-pregnane. The activities of 3α-HSO and 20α-HSO are reversible and control reductive and oxidative inter-conversions within the 4-pregnenes (inner ring of metabolites) or 5α-pregnanes (outer ring of metabolites). Identification of 5α-pregnan-3β-ol-20-one as a metabolite of all four cell types, indicates the presence of 3β-HSO activity, but at this time a full pathway of 3β-reduction/oxidation is not identified.