The Tag:ePDZ(Unk) entry in FlyBase represents an engineered PDZ domain that binds a C-terminal peptide epitope of Tag:LOVpep with high affinity, but where the particular engineered PDZ domain variant is not known or is not specified. Tag:ePDZ(Unk) can be used in combination with Tag:LOVpep to bring together two tagged proteins of interest in vivo in a light-dependent manner, for example to reconstitute a functional protein by bringing together complementary split system components, or to target a protein of interest to a particular subcellular location. In the dark, binding of Tag:ePDZ(Unk) and Tag:LOVpep is inhibited due to caging of the C-terminal epitope in Tag:LOVpep by the Jα helix binding to the LOV2 core. Upon blue light irradiation, unfolding of the Jα helix exposes the C-terminal peptide epitope in Tag:LOVpep, allowing it to bind Tag:ePDZ(Unk) and form a heterodimer (PMID:22388287).