Cytokine that acts as a physiological ligand for receptor tyrosine kinases LTK and ALK, leading to their activation. Cytokine-binding is sufficient to activate LTK. In contrast, ALKAL2-driven activation of ALK is coupled with heparin-binding to ALK. Stimulation of ALK signaling is involved in neural development and regulation of energy expenditure. Homodimer. Following interaction with receptor tyrosine kinase ALK, associates with the cell membrane, membrane-binding is required to activate ALK. Belongs to the ALKAL family.