CYP27B1 and neoplasm: This type of hypercalcemia was likely due to upregulation of the expression of Cyp27B1 in tumor cells, which encodes 1-α-hydroxylase, the enzyme responsible for converting 25-hydroxyvitamin D to the active hormone 1,25-dihydroxyvitamin D. Excess 1,25-dihydroxyvitamin D increases intestinal calcium absorption as well as bone resorption, leading to hypercalcemia.