DescriptionMammalian cell expression has become an important recombinant protein production system for a wide variety of applications because of the cells capacity for proper posttranslational modification and molecular structure assembly. In the past, mammalian expression has been optimized for secreted, glycosylated proteins. However, intracellular protein expression leaves yet another layer of complexity to be solved when expressing proteins in mammalian cells. There is a set of challenges that associates with intracellular protein expression due to the post-translational modifications involved with the bioactivity of the protein. The benefits to cytoplasmic protein production in mammalian cells, particularly for eukaryotic proteins, are very significant. Improvements in this area have been slow, however, due to limited development of the cell culture processes needed for low-cost and higher-throughput expression in mammalian cells. Here, we describe the development of a novel robust protein expression system using recombination cloning, suspension cells and stable lentiviral transfection to decrease the time and lower cost that can be combined to make mammalian cells much more amenable for routine recombinant protein expression.