No explanation has been available concerning the γ globulin defect in a unique family with one member whose serum was devoid of all the usual Gm genetic antigens. In the present study, it was found that this serum lacks ordinary γG1 and γG3 proteins and contains instead hybrid molecules of the type γG3-γG1. These were demonstrated most clearly by the precipitation of γG1 proteins with antisera specific for γG3 antigens. The analogy to the delta-beta chain hybrids, established for Lepore-type hemoglobins, was striking. An unequal homologous crossover involving mispairing of heavy chain cistrons would readily explain the deletion of genetic markers.