Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase of the insulin receptor superfamily. ALK-1 is typically expressed at low levels in regions of the developing central and peripheral nervous system. ALK-1 may be activated in cancer through multiple mechanisms. The most common mechanism is through the formation of a fusion protein from chromosomal translocations, as in the case of anaplastic large cell lymphoma (ALCL) and inflammatory myofibroblastic tumors. ALK-1 may also be amplified through mutation, as in neuroblastomas. Various solid tumors, such as non-small cell lung carcinoma (NSCLC) and brain cancers were also found to aberrantly express ALK-1. ALK-1 staining is present within both the nucleus and cytoplasm and is positive in about 60% of ALCL. ALK-1 protein expression by tumor cells is an independent prognostic factor that predicts a favorable outcome.