Consequently, there is an overwhelming case to be made for the clinical utility of measuring 24,25‐(OH)2D3 in hypercalcemic conditions such as idiopathic infantile hypercalcemia (IIH), where there is a mutation in the CYP24A1 gene that results in a defective CYP24A1 enzyme and little or no production of 24,25‐(OH)2D3 or other 24‐hydroxylated metabolites such as 25‐OH‐D3‐26,23‐lactone and 1,24,25‐(OH)3D3.(8) Since 24,25‐(OH)2D3 is the most abundant of these 24‐hydroxylated forms, its measurement has become the basis of the screening test for IIH in clinical practice. This evidence concerns the gene CYP24A1 and hypercalcemia, infantile.