Genetic variations are playing an increasing role in drug discovery, particularly in disease-specific drug target identification and in drug candidate profiling to predict drug response in genetically heterogeneous patient populations. Recently, genetic studies have successfully contributed to the identification of new susceptibility genes, disease mechanisms and potentially novel disease-specific drug targets for common diseases such as Type II diabetes, Crohn's disease, asthma and osteoporosis. Numerous variants of cytochrome P450 enzymes and the pregnane X receptor, recently associated with protein expression and altered catalytic activities, may prove to be of use in the future in drug candidate profiling.