Abstract: A system is provided an enzyme donor (“ED”) fused a surrogate of a mammalian protein of interest, where the fusion protein has the function of the natural protein. A vector is provided comprising a regulatory region functional in a mammalian host cell, a sequence encoding the ED joined to a multiple cloning site, an enzyme acceptor (“EA”) protein or enzyme acceptor sequence encoding such protein, and substrate for the enzyme formed by ED and EA.

Abstract: A system is provided for producing biologically active fusion proteins comprising a sequence encoding an enzyme donor (“ED”) sequence of fused in reading frame to a sequence encoding a surrogate of a mammalian protein of interest, where the fusion protein has the function of the natural protein. A vector is provided comprising a transcriptional and translational regulatory region functional in a mammalian host cell, a sequence encoding the ED joined to a multiple cloning site, an enzyme acceptor (EA) protein or enzyme acceptor sequence encoding such protein, that is complemented by the ED to form a functional enzyme, e.g. &bgr;-galactosidase, and substrate that is turned over by the enzyme to form a detectable substrate. Mammalian cells are employed that may be modified to provide specific functions, such as expression of the EA, overexpression of a protein of interest, etc. The system is used to monitor the fusion protein as a surrogate for the natural protein.