Blood is collected at baseline and on days 14, 30, 60, 90, 180, 270, and 360. Peripheral blood mononuclear cells for CYP24 genotyping, protein expression, enzyme activity, and splicing variants are analyzed by polymerase chain reaction (PCR), western blot, high performance liquid chromatography, and reverse transcriptase PCR, respectively. Serum is analyzed for vitamin D_3 metabolite levels (by radioimmunoassay), calcium (to monitor for hypercalcemia), and parathyroid hormone assays (to measure vitamin D effect).