Expression is highest in the heart with very low to almost undetectable levels in brain, skeletal muscle, stomach, pancreas, liver, kidney, small intestine, and uterus. At embryonic day 8.5 dpc, expressed in the developing heart with the strongest expression occurring in the inflow tract, especially in the left horn. At 9.5 dpc, expression is still detectable in the atria and ventricles, albeit at lower levels whereas strong expression remains in the inflow tract. From 10 dpc onwards, it is also expressed at low levels in somites, particularly in the myotome region, that gives rise to skeletal muscle. At 10.5 dpc, cardiac expression is no longer restricted to the inflow tract. At 14.5 dpc, it is expressed in the entire myocardium. Belongs to the steroid 5-alpha reductase family.