Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)In the present study, we evaluated and described the sensitivity of the stem cell marker B cell-specific moloney murine leukemia virus integration site 1 (Bmi-1) in identifying early lesions of carcinoma ex pleomorphic adenoma (CXPA). While invasive CXPAs are tumors with a prominent and easily recognizable malignant component, the identification of early carcinomatous changes in PA remains a diagnostic challenge due to the lack of objective morphological criteria. The immunohistochemical expression of Bmi-1 was assessed in both adenomatous and carcinomatous components of 9 CXPA cases at an early phase of histological progression (6 intracapsular and 3 minimally invasive) grouped according to the cellular differentiation as luminal (7 cases) or myoepithelial (2 cases). A selective nuclear expression of Bmi-1 was found exclusively in the malignant component of 8 cases (6 luminal type and 2 myoepithelial type), including intraductal carcinoma areas, except for 1 case in which scarce cells of the remnant PA were positive. Thus, Bmi-1 is expressed from the earliest morphologically detectable stages of PA malignant transformation. When faced with atypical features in PA, evaluation of Bmi-1 expression can provide more objective criteria for identification and diagnosis of early lesions of CXPA. This is applied to carcinomas with luminal or myoepithelial differentiation.9427 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [process 2012/00786-1]