ALK1 is a 200 KDa transmembrane receptor protein, which is normally expressed in cells of the developing and mature nervous system, such as the glial cells, neurons and endothelial cells. In the immunohistochemical setting the ALK-1 clone is useful to help identify a subgroup of ALK-positive anaplastic large cell lymphomas (ALCLs) (Pulford et al. Gascoyne et al.).

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