<unk>  is a zoosterol that functions in the serum as a cholesterol precursor, and is converted to pre-vitamin D3 in the skin, therefore functioning as provitamin-D3. The presence of this compound in human skin enables humans to manufacture vitamin D3 from ultra-violet rays in the sun light, via an intermediate isomer pre-vitamin D3. It is also found in the milk of several mammalian species. It was discovered by Nobel-laureate organic chemist Adolf Windaus.
7-Dehydrocholesterol
Tetrahydrofolic acid
Beta-carotene
Diiodotyrosine
Cholesteryl ester
Dihydrotachysterol
Hydroquinone
Tetrahydrobiopterin