Expression of ALK protein. (A) Staining for the intracellular portion of ALK protein (antibody ALK1) is seen as granular cytoplasmic staining associated with a dot-like reaction in the Golgi area. (B) The same antibody gives a diffuse cytoplasmic and nuclear labeling of a classical ALCL expressing the chimeric NPM-ALK protein. (C) Staining with an antibody (ALK-EC) against the extracellular portion of the ALK molecule shows a membrane associated labeling pattern. (D) A classical ALCL case expressing NPM-ALK is negative with this antibody (immunoperoxidase staining with nuclear counterstain).