A new type of selective models extending the possibilities of the existing methods of radiation hybrid mapping has been developed. These models are intended for mapping a "combined" group of genes. The genes are detected in a given clone by either direct molecular genetic methods or methods of isozyme analysis, which allow the gene to be detected indirectly, i.e., by the presence of its protein product. This type of selective models allows for both strict and nonstrict retention of the selective gene in the hybrid clone (the strictly and nonstrictly selective media, respectively). Asymptotic properties of the new type of models have been studied. It has been demonstrated that all of them are pseudo-Markovian and additive and may be used for mapping an unrestricted number of genes. The minimum numbers of genes being localized for which different variants of the models hold true have been estimated.