Calcitriol is the hormonally active metabolite of vitamin D with the addition of three hydroxyl groups. Its main biological effect is to increase the mineralization of bone and the uptake of calcium from the gut into the blood. Calcitriol activates osteoclasts by causing osteoblasts to release Receptor activator of nuclear factor kappa-B ligand (RANKL). It is also reported to exhibit antitumor properties by inhibiting the proliferation of a variety of carcinomas and leukemia cells.