Expression of STAT1 in ALK+ ALCL cell lines and patient samples. (A) Western blot studies revealed a relatively low expression level of STAT1 in 3 ALK+ ALCL tumors and 3 cell lines compared with that of normal peripheral blood T cells. Densitometry values shown were normalized to the β-actin bands. Most cases also had a low expression level of pSTAT1. (B) Immunohistochemical study results showed the expression of STAT1 in paraffin-embedded tissues derived from a representative benign tonsil (left, ×100 magnification), a representative ALK+ ALCL tumor (middle, ×400 magnification), and a representative ALK− ALCL tumor (right, ×200 magnification). In benign tonsils, the interfollicular T cells (white arrow) were strongly STAT1-positive, whereas the germinal center B cells (black arrow) were STAT1-weak. In the ALK+ ALCL tumor, the infiltrating T cells were strongly positive for STAT1, whereas the large lymphoma cells (red arrow) were virtually negative for STAT1. In the ALK− ALCL tumor, the large lymphoma cells (blue arrow) were strongly positive for STAT1.