We developed a method to identify gene x environment interactions (G x Es). To test this method in the simulated data (Problem 2, GAW11), we first identified an environmental factor (E1) that was associated with the simulated disorder. We stratified affected sibling pairs (ASPs) into two groups, those concordant for the presence of E1 and those concordant for the absence of E1. We then localized genes on chromosomes 3 and 5 using identity-by-descent (IBD) sharing rates among ASPs. Because the stratified IBD sharing rates are independent of the environmental factor if there is no G x E, we inferred the existence of a G x E near loci 3G44 and 3G45 by testing whether the proportion of ASPs sharing no alleles IBD differed among the two groups.