This invention relates to the production of 7-dehydrocholesterol, 25-hydroxy-7-dehydrocholesterol, and 25-hydroxy ergosterol in yeast such as Saccharomyces cerevisiae. It also related to various enzymes catalyzing the reduction of the double bond at position 24 of lanosterol, dimethyl zymosterol, methyl zymosterol, zymosterol, cholesta-7, 24-dienol, or cholesta-5,7,24-trienol; or the hydroxylation at position 25 of ergosterol, 7-dehydrocholesterol, cholesta-8-enol, and cholesta-7-enol. It also relates to various nucleic acids encoding cholesterol C25-hydroxylases and sterol Δ24-reductases and their use to produce and hydroxylate 7-dehydrocholesterol or ergosterol. It also relates to the yeast strains so produced, and methods of making these sterols comprising the steps of cultivation a transformed yeast cell, and harvesting the resulting sterol(s).