CYP2R1 and vitamin D deficiency: In humans, genetic mutation in the CYP2R1 gene causes an inherited form of vitamin D deficiency and rickets in children (3, 4), and genome-wide association studies have identified CYP2R1 gene variants as 1 of the key genetic determinants of low 25-hydroxyvitamin D (25-OH-D) levels (5, 6).