Basal-like tumors are a newly recognized estrogen receptor (ER) negative and HER2 negative breast cancer subtype that express basal epithelial genes and are associated with poor survival. Metaplastic carcinomas are thought to belong within the basal-like group. We have recently demonstrated that the small heat shock protein alphaB-crystallin is commonly expressed in basal-like tumors and contributes to their aggressive phenotype. The current study examined the rates and patterns of alphaB-crystallin expression in whole tissue sections of human breast, including normal tissue, proliferative lesions, in situ and invasive carcinomas (ER positive, HER2 positive, basal-like, and metaplastic cancers). In normal breast tissue, proliferative lesions and in situ carcinomas, alphaB-crystallin expression was restricted to the myoepithelial cell compartment of ductal and lobular units. Most basal-like and metaplastic carcinomas demonstrated cytoplasmic expression of alphaB-crystallin (81% and 86%, respectively). Conversely, no staining for alphaB-crystallin was observed in nonbasal-like (ie, ER positive or HER2 positive) breast carcinomas. Taken together, our results indicate that alphaB-crystallin is a sensitive (81%) and specific (100%) marker for basal-like breast carcinomas. Moreover, the high rates of expression of alphaB-crystallin in metaplastic breast carcinomas (86%) suggest that these tumors may represent a histologically distinctive subset of basal-like breast tumors with a similar underlying molecular etiology.