In hypercalcaemic patients, the parallel measurement of both vitamin D metabolites and calculation of the VMR allows distinguishing CYP24A1 deficiency from other causes of hypercalcaemia, such as vitamin D intoxication, granulomatous disease, Williams-Beuren syndrome or mutations of the SLC34A1 gene [18, 170]. This evidence concerns the gene CYP24A1 and hyperinsulinemic hypoglycemia, familial, 4.