Chromosome (Chr) 12 is a newcomer in the linkage genetics of the mouse. None of the classical linkage groups of the mouse map there, despite the suggested association of Va (LG XVI, now known to map to Chr 3) and the Chr 12 arm of the Rb(8.12)5Bnr Robertsonian chromosome (Cattanach and Moseley 1973). No genes were correctly assigned to Chr 12 until the late 1970s, when a combination of linkage genetics and somatic cell hybrid analysis placed the Aat-Igh linkage group (Taylor et al. 1974) at the distal end of the chromosome (Hengartner et al. 1978; Meo et al. 1980; D'Eustachio et al. 1980), and Arnheim and co-workers (1982) mapped a DNA length variant associated with the ribosomal RNA gene cluster on the chromosome to a site near the centromere. Perhaps because of its recent origin, the map of the chromosome depends to an unusually large extent on the analysis of backcross progeny segregating for multiple DNA markers (for example, Birkenmeier et al. 1988; Blank et al. 1988; Glaser et al. 1989; Seldin et al. 1989). As a result, it is straightforward to build an internally consistent linkage map that spans the chromosome. At the same time, the map is poor in welllocalized visible mutations. The 78 genes mapped to the chromosome, by a combination of linkage genetics, analysis of somatic cell hybrids, and in situ hybridization studies, are listed in Table 1. Four assumptions allow two conclusions to be drawn as to the expected size of Chr 12. Assuming a constant relationship between physical size of the chromosome at metaphase and genetic size, Chr 12, which represents 4.63% of the haploid metaphase chromosome length (Nesbitt and Francke 1973), should have a genetic length of 74 cM (assuming a haploid genome size of 1600 cM; Carter 1955). Assuming a constant relationship between physical size and