Abstract: The invention relates to a process for the selection from a gene library of a gene encoding an enzyme that is capable of catalyzing the conversion of a prodrug to its active drug form. The method comprises contacting a library of lysogenic bacteria with a prodrug that causes activation of bacterial RecA when converted to its active drug form. Activation of RecA causes lysis of the bacteria, so allowing separation of bacteriophage particles released into the medium, and their subsequent genotypic analysis to isolate nucleic acid molecules in the library that encode a desired prodrug-activating enzyme.