Anaplastic Lymphoma Kinase (ALK), also known as CD246, is a 220 kDa transmembrane glycoprotein that is primarily expressed in the developing nervous system. ALK contains one LDL Receptor (LDLR) class A domain, two meprin/A5-protein/PTPmu (MAM) domains, and a cytoplasmic kinase domain. Chromosomal translocation involving ALK and the nucleolar NPM protein result in a cytoplasmic fusion protein that exists as a dimer. The kinase domain in NPM-ALK is constitutively active and participates in cellular transformation. ALK translocations are found in a variety of tumors. Within the extracellular domain, human and mouse ALK share 88% amino acid sequence identity.