G protein-coupled receptors (GPCRs) are central mediators of mammalian cells’ ability to sense and respond to their environment. The 813 human GPCRs are the largest class of membrane receptors, are central mediators of cell physiology, and are the target of ~34% of all U.S. Food and Drug Administration (FDA)-approved drugs and ~60% of prescriptions. Here we will leverage recent advances in DNA synthesis, genome editing, next-generation DNA sequencing, and informatics develop a platform to build thousands of individual mutations to GPCRs and experimentally characterize their effects in a novel assay that can be done in a simple pooled format in human cell lines. We will develop methodologies to characterize how these mutational libraries functionally signal through the two major pathways that GPCRs signal through, the cyclic AMP and calcium signaling pathways. The profiles will give us the ability to better understand the functional consequences of genetic variation in GPCRs and how they might be impacting diseases and drug responses. In addition, analysis of these mutational profiles will give us insights on how to target these receptors for making new drugs. The technologies developed here should be broadly and directly applicable to the vast majority of GPCRs, as well as other important classes of drug targets where function can be assessed by transcriptional reporters in human cell lines.