VDR and type 2 diabetes mellitus: Decreased vitamin D level, expression of the vitamin D receptor (VDR), and deregulation of the function of vitamin D-metabolizing enzymes are all related to serious diseases such as autoimmune diseases, neurologic disorders including schizophrenia, type 2 diabetes mellitus, increased mortality, multiple sclerosis, and others (reviewed in [8,9,10,11,12,13,14,15]).