In electronic circuits, interconnected transistors produce a binary output in response to one or more inputs. Here, the authors used genetically interconnected transcriptional and translational regulators to perform such logical functions in mammalian cells. Human embryonic kidney cells were transfected with combinations of three types of plasmids. One type encoded synthetic transcription factors (STFs), engineered so that specific exogenous molecules (inputs) could suppress STF promoter binding. The second type expressed RNA-binding proteins (RBPs) under the control of an STF promoter (so RBP expression could be repressed by an STF-specific input). The third type expressed fluorescent proteins, and this expression was regulated by two inputs, acting through an STF promoter and RBP-mediated translation inhibition, respectively. Depending on the plasmid combination, a binary output (presence or absence of fluorescence) was produced in response to specific inputs. As these synthetic genetic circuits perform complex logical functions, they could be used to generate biocomputers.