The ability to obtain essentially unrestricted quantities of a cloned protein via expression in a bacterial, fungal, or eukaryotic cell expression system facilitates the structural and functional characterization of that protein. The rapidly increasing availability of diverse expression vectors and complementary cellular systems will eventually allow for the heterologous expression of any protein (1 -3 ). Heterologous protein expression with concomitant radiolabeling provides an attractive means to produce a readily detectable, biochemically active form of the protein without the potential chemical modifications that may arise when a radioactive or other detectable marker is covalently linked to it.