ALKAL1 (ALK and LTK ligand 1) is a cytokine that serves as a physiological ligand for the receptor tyrosine kinase (RTK) LTK 1. Monomeric ALKAL1 binds to LTK, inducing LTK homodimerization and subsequent activation of the kinase domain 21. Unlike its homolog ALKAL2, ALKAL1 does not function as a potent physiological ligand for ALK 1. The ALKAL1-LTK interaction activates downstream signaling cascades including ERK1/2 and ERK5 pathways, promoting neuron projection development 1. Beyond canonical LTK activation, ALKAL1 exhibits broader immunological functions. In the tumor microenvironment, ALKAL1—a target gene of the aryl hydrocarbon receptor (AhR)—facilitates MerTK phosphorylation on macrophages, enhancing their phagocytic activity and promoting immunosuppressive polarization, thereby contributing to melanoma progression and immunotherapy resistance 3. In cancer contexts, ALKAL1 upregulation correlates with poor prognosis; colorectal cancer studies demonstrate that ALKAL1 promotes migration and invasion through activation of Sonic hedgehog (SHH) signaling 4. Clinically, ALKAL1's role in nociceptor sensitization suggests therapeutic potential; ALK inhibitors (crizotinib, lorlatinib) that block ALKAL1/ALK signaling reverse pain hypersensitivity in inflammatory and neuropathic conditions 5.