Fat-soluble prohormone, the two major forms of which are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is produced in the skin when it is exposed to sunlight (UVB); vitamin D3 can be made in the skin at least two times per week after only 10 to 15 minutes of sun exposure to the face, arms, hands, or back without sunscreen. Vitamin D3 is hydroxylated in the liver and stored as 25-hydroxyvitamin D3 (calcidiol); calcidiol is again hydroxylated in the kidney to the main biologically active hormone 1,25-dihydroxyvitamin D3 (calcitriol). Vitamin 25-hydroxyvitamin D3 (25-OHD) is the commonly accepted serum marker for vitamin D status.