ALK and neuroblastoma: Of note, another RTK namely ALK (anaplastic lymphoma kinase) had been considered as an orphan receptor with no endogenous ligands until recently when a cytokine termed ALKAL (ALK and LTK ligand; also termed FAM150 or augmentor) was proposed as an in vivo ligand of the ALK family of RTKs in human neuroblastoma cells [186–188].