An internal ribosome binding site can be used to select for homologous recombinants at an immunoglobulin heavy-chain locus.

The encephalomyocarditis virus (EMCV) leader sequence is responsible for efficient, cap-independent translation initiation from the viral RNA. It has been used to increase the expression of internal coding regions on polycistronic mRNA encoded by recombinant DNA constructs. We have designed a sequence-replacement-type vector for targeting to immunoglobulin heavy-chain loci in hybridoma cells. Homologous recombination of this vector introduces a human gamma 1 constant-region sequence linked to the EMCV leader and a neomycin phosphotransferase (neo) gene. The resulting cells express a bicistronic mRNA encoding at the 5' end a chimeric murine VDJH-human C gamma 1 heavy chain, followed by neo linked to the internal ribosome binding site provided by the EMCV leader. These homologous recombinants express the chimeric heavy chain at levels equivalent to the heavy chain in the parental hybridoma. This strategy of using an EMCV-neo cassette to obtain efficient selectable marker gene expression has potential application to a range of gene targeting vectors.