Probable ligand that binds to the torso receptor. Implicated in a receptor tyrosine kinase signaling pathway that specifies terminal cell fate. Is a component of the vitelline membrane (VM, the inner layer of the eggshell). Restricted to specialized categories of follicle cells localized at the poles of the egg chamber. Expressed both maternally and zygotically. At early blastoderm stage, forms a symmetrical concentration gradient at the poles on the surface of the devitellinized embryo.