The wild-type anaplastic lymphoma kinase (ALK) protein is a 200kDa transmembrane receptor tyrosine kinase. Its expression is restricted to a few scattered cells in the nervous system (some glial cells and neurons, and a few endothelial cells and pericytes. The hybrid gene,NPM-ALK, created by the t(2;5)(p23;q35) chromosomal translocation encodes part of the nucleolar phosphoprotein, nucleophosmin (NPM), joined to the entire cytoplasmic portion of the anaplastic lymphoma kinase (ALK) receptor tyrosine kinase. As a consequence, the ALK gene comes under the control of the NPM promoter, which induces a permanent and ubiquitous transcription of the NPM-ALK hybrid gene, resulting in the production of a 80kDa NPM-ALK chimeric protein. This translocation is found in anaplastic large cell lymphomas (ALCL). Reportedly, expression of ALK indicates a better prognosis. Approximately 5%-10% of non-small cell lung carcinomas also express ALK protein producing a cytoplasmic staining pattern. This MAb also reacts with blood vessels that serves as an internal positive control.