The reconstructed ancestral boreoeutherian karyotype, derived from synteny analyses of human, mouse, rat, cow, dog, opossum and chicken genome sequences, and based on the identified orthology blocks, is depicted in Additional file 1. The ideograms represent the 22 autosomal syntenic groups of the ancestral genome as well as the ancestral X chromosome. The orthologies to the human genome are given for entire chromosomes below each chromosomal ideogram and to the right of the ideograms for the individual conserved segments. For conserved segments representing portions of human chromosomes, the positions of the boundaries of the orthologous segments in the human genome are listed above the ideograms in Mb. Boundaries in agreement with previous findings, and based on comparative cytogenetics, are given in black whereas the boundaries refined in this study are indicated in blue. The sizes of the chromosomal ideograms reflect the approximate size ratios of the euchromatic orthologous segments in the human genome. The association of the segment orthologous to HSA10p with segments orthologous to HSA12 and HSA22 is based on comparative chromosome painting data from carnivores , hedgehog, several afrotherian [10, 60] and xenarthran [55, 56] species as well as the opossum genome sequence . The comparative chromosome painting data for afrotherian and xenarthran species further indicate that the syntenic groups of the ancestral boreoeutherian karyotype are identical with those of the eutherian karyotype.