Impaired expression of endogenous Bcl-2 in the EBV-infected NK leukemia cells. Analysis of Bcl-2 expression in either normal NK cells, NK leukemia cells, or NK cells from chronic NK lymphocytosis on immunohistochemical (upper panels) and flow cytometric (lower panels) analyses. On both assays, endogenous Bcl-2 was undetectable in NK leukemia cells, although NK cells from normal controls and a case of chronic NK lymphocytosis constitutively expressed Bcl-2. On the immunohistochemical method, Bcl-2 is visualized as diffuse red staining in cytoplasm in normal NK cells and chronic NK lymphocytosis (upper left and right panels). Results are summarized in Table 2.