Bone formation requires phosphate to calcify the osteoid produced by osteoblasts, Pit-1, a natrium-phosphate cotransporter, is expressed in osteoblasts and its expression levels are regulated developmentally and also by hormones and cytokines. Another type of phosphate transporter is expressed in osteoclasts and its function is required for bone resumption. These observations suggest that phosphate transport into the bone cells may play a role in regulation of bone formation and resorption in vivo and in the pathological situation in patients with bone diseases.