CYP24A1 and hypercalcemia, infantile: Two forms of IIH have been distinguished based on genetics and mechanisms: in Hypercalcemia, Infantile 1 (HCINF1; OMIM 143880) biallelic loss-of-function mutations in CYP24A1 impair inactivation of 1,25(OH) 2D; in HCINF2 (OMIM 616963) biallelic loss-of-function mutations in SLC34A1 lead to excessive synthesis of 1,25(OH)2D.