Cytokine that acts as a physiological ligand for receptor tyrosine kinase LTK, leading to its activation (PubMed:25331893, PubMed:26418745, PubMed:26630010, PubMed:34646012, PubMed:34819673). Monomeric ALKAL1 binds to LTK, leading to LTK homodimerization and activation (PubMed:34646012, PubMed:34819673). In contrast to ALKAL2, does not act as a potent physiological ligand for ALK (PubMed:26418745, PubMed:34646012). Following interaction with receptor tyrosine kinase LTK, associates with the cell membrane, membrane-binding is required to activate LTK. Widely expressed with highest levels in thyroid and moderate levels in stomach, trachea, small intestine, prostate and brain. Belongs to the ALKAL family.