Mus poschiavinus is a species of mouse found in Val Poschiavo, Switzerland. Its genome is organized in a karyotype of 26 chromosomes, instead of the usual 40. This includes 7 pairs of metacentrics, 5 pairs of acrocentrics, and the acrocentric X and Y (Cattanach et al., 1972). Due to the tobacco darkening allele (Mc1rE-tob) in this population, Mus poschiavinus mice have a black coat color that becomes agouti in the flanks after about 8 weeks of age.
In the late 1960s Alfred Gropp bred a Swiss NMRI female to a wild trapped male from Val Poschiavo, Switzerland and sent the F1 offspring to Frank Ruddle at Yale University who shipped these same F1 hybrids to Tom Roderick at The Jackson Laboratory. These F1 hybrids were sibling intercrossed and several inbred lines were generated from inbreeding of this population. These lines were selected for the presence Robertsonian translocations with the intention of generating inbred strains carrying a single Robertsonian or a defined set of Robertsonians. At F3 two lines separated: line PH was inbred to produce a line homozygous for Rb(16.17)7Bnr; and line PI was further separated at generation F4 into lines PAD and PAC. The PAC line was maintained by sibling intercross and the offspring of the F7 sibling intercross were separated into several further sublines, including PJG and PJI, which were each sibling intercrossed for one generation then cousin intercrossed before again being maintained by sibling intercross. In 1977 this line reached generation F17 and was designated subline N and in 1983 it reached generation F32. While this line was selected to be maintained homozygous for Rb(11.13)4Bnr, it had been maintained segregating for Mc1rE-tob and Tyrc, but at some point it became fixed homozygous for Tyrc. In 1990 when this inbred reached generation F73 an attempt was made to cryopreserve embryos but this was not successful. The strain remained on the shelf in the care of Norman Hawes & Bo Chang and they discovered the retinal degeneration 3 mutation (Rd3rd3) in it. Because this strain was inbred from an outbred population crossed to a wild male it has a unique genomic makeup compared with other inbred strains and even with other inbred strains derived from the same ancestral F1 population.