A steroid hormone produced in the skin when exposed to ultraviolet light or obtained from dietary sources. The active form of cholecalciferol, 1,25-dihydroxycholecalciferol (calcitriol) plays an important role in maintaining blood calcium and phosphorus levels and mineralization of bone. The activated form of cholecalciferol binds to vitamin D receptors and modulates gene expression. This leads to an increase in serum calcium concentrations by increasing intestinal absorption of phosphorus and calcium, promoting distal renal tubular reabsorption of calcium and increasing osteoclastic resorption. OR A rare cancer that forms in the bile ducts. A bile duct is a tube that carries bile (fluid made by the liver) between the liver and gallbladder and the small intestine. Intrahepatic cholangiocarcinoma is found inside the liver. Extrahepatic cholangiocarcinoma is found outside the liver. Also called bile duct cancer.