Original Articlep63 Expression in Breast Cancer: A Highly Sensitive and Specific Marker of Metaplastic CarcinomaKoker, Meryem M MD; Kleer, Celina G MDAuthor Information From the Department of Pathology, University of Michigan Medical Center, Ann Arbor, MI. Supported in part by NIH grant K08 CA090876-01, Department of Defense grants DAMD17-02-1-0490 and DAMD17-02-1-0491, and a John and Suzanne Munn Award from the University of Michigan Cancer Center (to C.G.K.). Presented at the United States and Academy of Pathology Meeting (USCAP), Vancouver, BC, March 6–12, 2004. Reprints: Celina G. Kleer, MD, Department of Pathology, 2G332 University Hospital, 1500 E. Medical Center Drive, Ann Arbor, MI 48109-0054 (e-mail: firstname.lastname@example.org). The American Journal of Surgical Pathology: November 2004 - Volume 28 - Issue 11 - p 1506-1512 doi: 10.1097/01.pas.0000138183.97366.fd Buy SDC Metrics Abstract p63, a member of the p53 gene family, is involved in cellular differentiation and is expressed in the nuclei of myoepithelial cells of normal breast ducts and lobules. Although p63 has been reported in metaplastic carcinomas of the breast, its expression pattern in breast carcinomas and sarcomas has not been fully characterized, and its potential diagnostic utility has not been defined. In this study, we determined p63 expression in a large number of breast carcinomas, including metaplastic carcinomas, and in Phyllodes tumors and sarcomas. We examined 189 invasive breast carcinomas, including 15 metaplastic carcinomas, as well as 10 Phyllodes tumors, and 5 pure sarcomas of the breast for pattern and intensity of p63 staining using an anti-p63 antibody (clone 4A4, Neomarkers). p63 was strongly expressed in 13 of 15 metaplastic carcinomas (86.7%). p63 was positive in all the metaplastic carcinomas with spindle cell and/or squamous differentiation (12 of 12), and in 1 of 3 metaplastic carcinomas with cartilage foci. In stark contrast, only 1 of 174 (0.6%) nonmetaplastic invasive carcinomas was positive for p63. All Phyllodes tumors and sarcomas were consistently negative for p63 expression. The sensitivity and specificity of p63 as a diagnostic marker for metaplastic carcinoma was 86.7% and 99.4%, respectively. We propose the inclusion of p63 as part of the diagnostic workup of challenging spindle cell tumors of the breast as a highly specific marker for metaplastic carcinomas. © 2004 Lippincott Williams & Wilkins, Inc.