A new PXO set of RIS represents a fixed F2 population derived from polydactylous (P) congenic strain SHR.Lx and oligodactylous (O) RI strain BXH2. The PXO strains were derived as a complementary set to current RIS (HXB, BXH) of the laboratory rat. All PXO strains are homozygous in the Lx allele and express different morphological phenotypes of the polydactyly-luxate syndrome (PLS) due to variable combinations of Lx modifying genes of either SHR or BN origin. The SDP is being built up by genotyping polymorphic microsatellite markers and several gene polymorphisms. The markers were ordered according to data from public mapping resources such as the Rat Genome Database (rgd.mcw.edu) and current SDP of the other RI strain sets (HXB, BXH). The resulting map corresponding to the common SDP of HXB, BXH RIS sets consists of 448 markers, from which 261 were proven to be polymorphic in the PXO set. The SDP of PXO strains with polymorphic markers arranged in approximately 5 cM intervals is ready for the association analysis and interval mapping in interconnection with the SDP of HXB/BXH strains.