In the patient with low circulating levels of 25(OH)D3 and symptoms of vitamin D deficiency, a study found a transition mutation in exon 2 of the CYP2R1 gene was found, which leads to the substitution of Proline Leucine at amino acid 99 in the CYP2R1 protein and abolishes enzymatic activity of vitamin D 25-hydroxylase, indicating that genetic variation in this gene can affect 25(OH)D synthesis [20]. The gene discussed is CYP2R1; the disease is vitamin D deficiency.