Ancestral karyotype of the genus Mus with location of breakpoints and rDNA clusters . All chromosomes are acrocentric. The segments orthologous to the house mouse chromosomes are indicated at the right of each chromosome; p, m, d refer to the proximal, median and distal segments of the chromosome, and a and b to unidentified subchromosomal segments. The green arrows point to breakpoints and the red arrowheads to rDNA clusters. The insert shows a schematic acrocentric chromosome with the two possible locations of rDNA clusters (red): (i) pericentromeric i.e. adjacent to the centromere (blue) and (ii) subtelomeric i.e. close to the distal telomere (black).