a hormone that can be produced by tumors, especially of the squamous cell type; massive overproduction can lead to hypercalcemia and other manifestations of hyperparathyroidism. PTHrP exerts a biologic action similar to that of parathyroid hormone (PTH), acting through the same receptor, which is expressed in many tissues but most abundantly in kidney, bone, and growth plate cartilage. It apparently has significant actions during development, but it remains uncertain whether PTHrP circulates at all or has any function in normal human adults. The structure of the gene for human PTHrP is more complex than that of PTH, and varying molecular forms exist, including proteins of 141, 139, and 173 amino acids, which share a significant homology with parathyroid hormone.