Approval for this study was obtained from the University of California, San Francisco (UCSF), Committee on Human Research. A database maintained by the UCSF Dermatopathology Service was used to identify primary and referral patients diagnosed as having ASC by means of standardized microscopic criteria during the period of January 1, 1996, through December 31, 2006. Key epidemiologic information was extracted from pathology requisition slips. Cases were excluded if the pathological diagnosis of the biopsy specimen was equivocal or the specimen was unavailable for review. All cases were evaluated by a single board-certified dermatopathologist (T.M.) for histopathological features and immunohistochemical characteristics. Immunostaining was performed with antibodies against high-molecular-weight keratin 903 (clone β E12; dilution, 1:50) (Enzo Life Sciences, Farmingdale, New York), carcinoembryonic antigen (polyclonal; dilution, 1:1000), cytokeratin 7 (clone OV-TL; dilution, 1:1000), epithelial membrane antigen (clone E29; dilution, 1:100), cytokeratin 20 (clone K20.8; dilution, 1:50), thyroid transcription factor 1 (clone 8G7G3/1; dilution, 1:200), epidermal growth factor receptor, prostate-specific antigen (dilution, 1:200), progesterone receptor (clone A0098; dilution, 1:100), and estrogen receptor (clone ID5; dilution, 1:280) (all from DakoCytomation, Carpinteria, California). Mucicarmine and von Kossa staining (American Master Tech, Lodi, California) was performed in select cases.