Cytokine that acts as a physiological ligand for receptor tyrosine kinase LTK, leading to its activation. Monomeric ALKAL1 binds to LTK, leading to LTK homodimerization and activation. In contrast to ALKAL2, does not act as a potent physiological ligand for ALK. Following interaction with receptor tyrosine kinase LTK, associates with the cell membrane, membrane-binding is required to activate LTK. Belongs to the ALKAL family.