Cytokine that acts as a physiological ligand for receptor tyrosine kinases LTK and ALK (PubMed:29078341, PubMed:29317532). Required for neural crest cell differentiation and iridophore development during embryonic iridophore development and adult stripe development by acting as a receptor for LTK (PubMed:29078341, PubMed:29317532). Homodimer. Expressed at high level in the notochord and iridophore stripes of the trunk, as well as in the eye and swim bladder. Fishes show reduced dense iridophores in the light stripes of the trunk, particularly in the anterior region, resulting in irregular interruptions of melanophore stripes (PubMed:29078341, PubMed:29317532). Fishes lacking alkal1, alkal2a and alkal2b are embryonic lethal and display total loss of iridophores (PubMed:29317532). Belongs to the ALKAL family.