The isolation, purification, and cloning of the mitochondrial P450 enzyme catalyzing not only the 27-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol and cholestane-3 alpha, 7 alpha, 12 alpha-triol, but also the 25-hydroxylation of vitamin D3 are reviewed. The sterol hydroxylase was shown to