Abstract: The present invention relates to vectors useful for transforming a lymphoid cell line to glutamine independence. The vectors comprise an active glutamine synthetase (GS) gene as well as a heterologous gene of interest to be expressed. The preferred embodiments encompass vectors wherein the heterologous gene is expressed from a relatively strong promoter and the GS gene is expressed from a relatively weak promoter. In one example, the heterologous gene is operatively linked to the hCMV-MIE promoter and the GS gene is operatively linked to the SV40 early region promoter.