Staining of paraffin sections of an ALK-positive ALCL (APAAP technique). (Upper) The reactivity of the same area of lymph node with antibodies to the C-terminal epitope (antibody NPMc) and to the N-terminal region (antibody NA24) is seen at low power (above) and high power (below). The tumor cells surround a residual area of normal B cells (asterisks). A clear difference between the labeling patterns of the two antibodies is seen even at low magnification, accounted for by the reactivity of the anti–N-terminal antibody with tumor cell cytoplasm. Scattered mitotic cells showing diffuse cytoplasmic labeling are seen in both sections (arrowed) and can be picked out in the low-power view of the immunostaining for the C-terminus. (Lower) The appearance of the tumor in sections stained by hematoxylin and eosin and for ALK protein is shown at low power (left) and high power (right).