Cytokine that acts as a physiological ligand for receptor tyrosine kinases LTK and ALK (PubMed:29078341, PubMed:29317532). Required for iridophore development in the adult eye by acting as a receptor for LTK (PubMed:29078341). Expressed at low level in the notochord and iridophore stripes, the eye and the swim bladder. Fishes show reduced iridophores in eyes and operculum, with no detectable phenotype in the trunk (PubMed:29317532). Fishes lacking both alkal1 and alkal2b show a complete loss of eye iridophores (PubMed:29317532). Fishes lacking alkal1, alkal2a and alkal2b are embryonic lethal and display total loss of iridophores (PubMed:29317532). Belongs to the ALKAL family.