VDR and rickets: VDDR2a (OMIM 277440) is due to biallelic loss-of-function mutations in the gene encoding the vitamin D receptor (VDR; 601769) on chromosome 12q13.11, and therefore represents a bona fide form of tissue resistance to vitamin D. As many patients with this disorder are unable to respond to any form of vitamin D, some have suggested that VDDR2 may be more appropriately described by the terms hereditary 1,25 (OH)2D-resistant rickets (HVDRR), hereditary resistance to 1,25 (OH)2D, or even pseudovitamin D-deficiency, type iia (PDDR IIA).