We are working to enable the engineering of integrated biological systems. Specifically, we would like to be able to build systems using standard parts that, when combined, have reliable and predictable behavior. Here, we define standard characteristics for describing the absolute physical performance of genetic parts that control gene expression. The first characteristic, PoPS, defines the level of transcription as the number of RNA polymerase molecules that pass a point on DNA each second, on a per DNA copy basis (PoPS = Polymerase Per Second; PoPSdc = PoPS per DNA copy). The second characteristic, RiPS, defines the level of translation as the number of ribosome molecules that pass a point on mRNA each second, on a per mRNA copy basis (RiPS = Ribosomes Per Second; RiPSmc = RiPS per mRNA copy). In theory, it should be possible to routinely combine devices that send and receive PoPS and RiPS signals to produce gene expression-based systems whose quantiative behavior is easy to predict. To begin to evaluate the utility of the PoPS and RIPS framework we are characterizing the performance of a simple gene expression device in E. coli growing at steady state under standard operating conditions; we are using a simple ordinary differential equation model to estimate the steady state PoPS and RiPS levels. / Poster presented at the 2005 ICSB meeting, held at Harvard Medical School in Boston, MA.