Drug targets are generally proteins. Nucleic acids are possible drug targets, and approaches such as RNA interference (RNAi) may indeed have advantages over proteins, but they have not so far been developed extensively. Protein targets consist mainly of enzymes, regulatory molecules such as cytokines, receptors for hormones or other regulatory molecules, transporters, and ion channels. Proteins that interact with small molecules are considered the most promising targets, because it is often possible to design or discover small molecule activators or inhibitors based on the natural ligands. Such substances are relatively straightforward to develop as drugs. Overall, enzyme targets account for over 50% of marketed drugs, and GPCR targets represent more than 20%, including 23 of the 100 best-selling drugs in the US. Hence the other target classes between them make up the remaining 25-30% of marketed drugs. Aberrant signal transduction plays a key part in the pathology of many serious diseases including cancer, inflammatory, cardiovascular, metabolic, and neuropsychiatric diseases. Proteins involved in signal transduction pathways therefore represent important drug targets. Three prominent superfamilies of proteins involved in signal transduction are: GPCRs, protein kinases, and nuclear receptors.