The extracellular calcium (Ca2+o)-sensing receptor (CaR) plays a critical role in maintaining Ca2+o homeostasis in mammals by virtue of its presence in parathyroid gland and kidney. The breast is well recognized as a Ca(2+)-handling organ, and the effects of altering Ca2+o on the proliferation of breast epithelial cells are well documented. To date there are no data regarding the expression and localization of CaR in breast tissue. In the present study, we assessed the distribution of CaR messenger ribonucleic acid (mRNA) and protein in normal and fibrocystic human breast tissue as well as in ductal carcinoma of the breast using RT-PCR, Northern analysis, and immunohistochemistry with CaR-specific antisera. In all tissues, RT-PCR performed using sense and antisense primers based on the sequence of the human parathyroid CaR complementary DNA amplified a product of the size expected (425 bp) for genuine CaR transcripts. Nucleotide sequencing of RT-PCR products confirmed more than 99% homology with human parathyroid CaR complementary DNA. Although insufficient quantities of mRNA were isolated from normal and fibrocystic tissue for Northern analysis, a single 5.2-kb CaR transcript was expressed in malignant breast tissue similar to the major CaR transcript in human parathyroid. Localization of CaR protein by immunohistochemistry showed specific CaR staining of the ductal epithelial cells of the breast in all three tissue types. These findings indicate the presence of CaR mRNA and protein in the breast, providing indirect evidence that the CaR may have some role(s) in the control of Ca2+ transport, epithelial cell proliferation, and/or other processes in normal and abnormal breast tissue.