The invention provides a new expression system comprising a mammalian selectable marker that promotes desirable post-translational modifications of glycoproteins. In particular, the invention includes methods and compositions for optimal recombinant protein expression in mammalian cells by employing a selection marker system based on GPT genes of mammalian origin. The invention includes methods that facilitate selectivity and enhanced expression copies as well as protein yield of recombinant proteins in mammalian cells, and methods of using GPT expression systems.