PTH and Hypercalcemia: Second, several lines of evidence implicate rs1570669 (CYP24A1) in the vitamin D pathway: its association with serum calcium and PTH concentrations, its selective expression in the proximal tubule where 1,25(OH)2-D metabolism occurs, and that loss-of-function CYP24A1 mutations cause vitamin D-induced hypercalcemia in children (idiopathic infantile hypercalcemia).