Lumisterol is a compound that is part of the vitamin D family of steroid compounds. It is the (9β,10α) stereoisomer of ergosterol and was produced as a photochemical by-product in the preparation of vitamin D1, which was a mixture of vitamin D2 and lumisterol. Vitamin D2 can be formed from lumisterol by a electrocyclic ring opening and subsequent sigmatropic hydride shift.