The present invention relates to the general field of recombinant expression of polypeptides in animal cell culture. More particularly, the invention concerns improved selection in cells of recombinantly engineered vectors designed to express polypeptides. It relates more specifically to a method of producing and isolating a heteromeric immunoglobulin heavy and light chain complex, the method comprising transfecting cells with a vector comprising a first nucleic acid encoding a first immunoglobulin heavy or light chain, wherein the transcription of said first nucleic acid is operably linked to transcription of a second nucleic acid encoding a first subunit of a selectable marker, and further comprising a third nucleic acid encoding a second immunoglobulin light or heavy chain wherein the immunoglobulin light and heavy chains are capable of associating to form a heteromeric complex, wherein the transcription of said third nucleic acid is operably linked to transcription of a fourth nucleic acid which encodes a
