Vectors incorporating a combination of promoters driving selectable marker expression

A mammalian host cell for expressing an antigen binding protein having three or four different chains comprising a first expression vector and a second expression vector, wherein:

• (a) the first expression vector comprises nucleotide sequences encoding 1) a first chain and a second chain, wherein the first chain is operably linked to a first promoter, the second chain is operably linked to a second promoter or an IRES sequence or is linked to the first chain through a linker sequence;
• and 2) a first selectable marker operably linked to a third promoter;
• and
• (b) the second expression vector comprises nucleotide sequences encoding 1) a third chain and a fourth chain, wherein the third chain is operably linked to a fourth promoter, the fourth chain is operably linked to a fifth promoter or an IRES sequence or is linked to the third chain through a linker sequence;
• and 2) a second selectable marker operably linked to a sixth promoter,
• wherein the first, second, third and fourth chains are selected from the group consisting of a heavy chain, a light chain, an antibody heavy chain fusion, an antibody light chain fusion, an ScFv, and an ScFv-Fc, wherein two of the chains may be identical;
• and
• wherein the sixth promoter is different from the third promoter.