Anaplastic lymphoma kinase was first described in 1994 as the NPM-ALK fusion protein that is expressed in the majority of anaplastic large-cell lymphomas. ALK is a receptor protein-tyrosine kinase that was more fully characterized in 1997. Physiological ALK participates in embryonic nervous system development, but its expression decreases after birth. ALK is a member of the insulin receptor superfamily and is most closely related to leukocyte tyrosine kinase (Ltk), which is a receptor protein-tyrosine kinase.