beta galactosidase complementation Beta-galactosidase activity can be used to monitor the interaction of chimeric proteins. Pairs of inactive beta gal deletion mutants are capable of complementing to restore activity when fused to interacting protein partners. Critical to the success of this system is the choice of two poorly complementing mutant moieties, since strongly complementing mutants spontaneously assemble and produce functional beta-gal activity detectable in absence of any fused protein fragment. PSI-MI beta galactosidase MI:0010 protein complementation assay