We next tested whether the effects of PTHrP on tumor cell growth and differentiation in vivo depended on signaling through the PTH1R by engineering mice with MMTV-Cre-mediated disruption of the Pth1r gene in the setting of tetracycline-regulated PTHrP overexpression and MMTV-PyMT-mediated mammary tumorigenesis. This evidence concerns the gene PTH1R and neoplasm.