Calbindin is a calcium-binding protein belonging to the troponin C superfamily. It was originally described as a 27-kD protein induced by vitamin D in the duodenum of the chick. In the brain, its synthesis is independent of vitamin-D-derived hormones. Calbindin contains 4 active calcium-binding domains, and 2 modified domains that presumably have lost their calcium-binding capacity. The neurons in brains of patients with Huntington disease are calbindin-depleted.