Colorectal cancer risk is inversely associated with the vitamin D status of the patient and epidemiological studies suggest that vitamin D supplementation decreases colorectal cancer incidence.1,2 25-hydroxyvitamin D3 (25-D3, calcidiol) is the circulating form of vitamin D. It is hydroxylated at position C-1 by 25-hydroxyvitamin D3 1α-hydroxylase (CYP27B1) to form the most active vitamin D metabolite 1,25-dihydroxyvitamin D3 (1,25-D3, calcitriol). The gene discussed is CYP27B1; the disease is colorectal cancer.