The present invention provides a method of producing functional protein domains, which are fragments of primary translational products consisting of one or more distinct protein domains encoded by specific subsets of exonic sequences. The method is based on the integration, by a single homologous recombination event, of a regulatory unit into the eukaryotic gene coding for the primary translational product at the level of the intronic genomic region immediately adjacent to the exonic sequences which code for the functional protein domain.