The Tag:GPI(Unk) entry in FlyBase represents a glycosylphosphatidyl inositol (GPI) attachment signal where the species and/or gene of origin are unknown. If a tagged protein of interest is targeted to the secretory pathway using an N-terminal signal sequence, covalent attachment of a GPI anchor to the GPI attachment signal can result in the modified protein being anchored to the external side of the plasma membrane (PMID:7574493).