ALK and C/EBPβ immunostaining in ALCL. The 2 ALK-positive cases, with nuclear (top) and cytoplasmic (middle) expression, show strong nuclear positivity in most tumor cells for C/EBPβ. The ALCL, ALK-negative case (bottom) is negative for C/EBPβ; however, the reactive histiocytes are strongly positive and serve as internal control. IP stain, 400×. Images were otherwise acquired as explained in Figure 1.