A common approach for testing gene-environment interactions is with case-control studies. Good case control studies have standardized definitions of the phenotype under investigations, as well as the environment risk factors and susceptibility genes that increase/decrease risk. Cases and controls can be categorized as being genetically susceptible (S+) or not susceptible (S-). They can be further categorized as having been exposed (E+) or not (E-). This is the simplest approach for beginning to explore gene-environmental interactions.