Vectors and expression systems for producing recombinant proteins

An expression system comprising:

• a) a first vector comprising a first nucleic acid encoding a first polypeptide, wherein the transcription of the first nucleic acid is operably linked to the transcription of a second nucleic acid encoding a first fragment of glutamine synthetase fused to a first interaction domain, and
• b) a second vector comprising a third nucleic acid encoding a third polypeptide, wherein the transcription of the third nucleic acid is operably linked to the transcription of a fourth nucleic acid encoding a second fragment of glutamine synthetase fused to a second interaction domain,
• wherein the first polypeptide is capable of associating with the third polypeptide to form a heteromeric complex,
• wherein the first and second fragments of glutamine synthetase are generated by splitting the glutamine synthetase polypeptide between amino acid positions equivalent to D92/P93, A186/G187, or G187/V188 of SEQ ID NO:
• 1 according to sequence alignment, the first fragment and second fragment associate via the first and second interaction domains to provide a selectable activity, and
• wherein the expression system is capable of being transfected into mammalian cells and improving selection of the cells.