Calbindin (CaBP)-D28k and CaBP-D9k are cytosolic vitamin D-dependent calcium-binding proteins long thought to play an important role in transepithelial calcium transport. However, recent genetic studies suggest that CaBP-D28k is not essential for calcium metabolism. Genetic ablation of this gene in mice leads to no calcemic abnormalities. Genetic inactivation of the vitamin D receptor (VDR) gene leads to hypocalcemia, secondary hyperparathyroidism, rickets, and osteomalacia, accompanied by 90% reduction in renal CaBP-D9k expression but little change in CaBP-D28k. To address whether the role of CaBP-D28k in calcium homeostasis is compensated by CaBP-D9k, we generated VDR/CaBP-D28k double knockout (KO) mice, which expressed no CaBP-D28k and only 10% of CaBP-D9k in the kidney. On a regular diet, the double KO mice were more growth-retarded and 42% smaller in body weight than VDRKO mice and died prematurely at 2.5-3 months of age. Compared with VDRKO mice, the double KO mice had higher urinary calcium excretion and developed more severe secondary hyperparathyroidism and rachitic skeletal phenotype, which were manifested by larger parathyroid glands, higher serum parathyroid hormone levels, much lower bone mineral density, and more distorted growth plate with more osteoid formation in the trabecular region. On high calcium, high lactose diet, blood-ionized calcium levels were normalized in both VDRKO and the double KO mice; however, in contrast to VDRKO mice, the skeletal abnormalities were not completely corrected in the double KO mice. These results directly demonstrate that CaBP-D28k plays a critical role in maintaining calcium homeostasis and skeletal mineralization and suggest that its calcemic role can be mostly compensated by CaBP-D9k.It is well established that calcium homeostasis is primarily maintained by the vitamin D and parathyroid hormone (PTH) 1 endocrine systems. In this regard, the principal role of vitamin D is to regulate calcium transport in the intestine and kidney. Intestinal and renal distal tubular calcium transfer consists of a passive, concentration gradient-dependent paracellular transport and an active, ATP-dependent transcellular transport (1). The latter is largely regulated by vitamin D and is crucial when calcium supply is low. It is believed that the vitamin D-mediated transcellular calcium transport in the duodenum and renal distal tubules involves three sequential steps: entry, intracellular movement, and extrusion. Influx of luminal calcium into the epithelial cells is propelled by a steep electrochemical gradient across the apical membrane, saturable at high levels of luminal calcium, and mediated by apical calcium channels (2-4). Once in the cytosol, calcium binds to the vitamin D-dependent calcium-binding proteins, calbindins (CaBPs), which ferry calcium from the apical to the basolateral portion of the cell, where the Na ϩ /Ca 2ϩ exchanger and Ca 2ϩ -dependent ATPase on the basolateral membrane extrude calcium into extracellular fluids (5-7). It is believed that the apical to b...