Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase of the insulin receptor superfamily. ALK is typically expressed at low levels in regions of the developing central and peripheral nervous system.

ALK may be activated in cancer through multiple mechanisms. The most common mechanism is through formation of a fusion protein from chromosomal translocations, as in the case of anaplastic large cell lymphoma (ALCL) and inflammatory myofibroblastic tumors. ALK 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.

ALK staining is present within both the nucleus and cytoplasm, and are positive in about 60% of ALCL. ALK protein expression by tumor cells is an independent prognostic factor that predicts a favorable outcome.