Conservation of genomic organization in different mammalian species has long been recognized, but only recently has it been possible to examine these relationships systematically on a genome-wide scale in some detail. Mapping of several mammalian species in progressing rapidly, but by far the most detailed information is still to be found in the human and mouse databases. Perhaps the most important aspect of recent progress in genome mapping data. With mapping databases continuing to expand at a greater than linear rate, any attempt at a comprehensive comparative map is doomed to be out of date by the time it is published. However, we feel that it is valuable to provide a summary that is as nearly up to date as possible. We have made a particular effort to include recent human physical mapping data and to identify those mouse genes that have been well-mapped with respect to each other by virtue of having been examined in the same cross. As the human-mouse comparative map becomes more dense, it is not surprising that the observed number of conserved linkage groups continues to increase. Nadeau et al. placed 425 loci on both maps, which delineated over 100 conserved linkage groups. Copeland etmore » al. put a total of 917 markers on both the human and the mouse maps, marking 101 segments of conserved linkage groups. In the present summary, we have placed 1416 loci, and these define at least 181 different conserved linkage groups. 47 refs., 1 fig.« less