An extra segment in chromosome 1 between bands C5 and D has been found in wild mouse populations. Its size varies between 6.1 and 30.1 of the length of a standard chromosome 1. It differs among individuals and populations but is constant in a genetically homogeneous line. According to its staining properties and variation in length it is a homogeneously staining region (HSR), a kind of segment otherwise found only in cell lines under strong selection and in tumor cells. G-banding gives a homogeneous staining of medium intensity. With C-banding, staining is positive, though lighter than that of centromeric heterochromatin. Fluorescence is dull with Hoechst 33258 and bright with mithramycin. The extra segment does not contain mouse satellite DNA sequences in any quantity detectable by in situ hybridization. Such an extra segment was found in several European populations of mice from Spain to Russia. It is carried through the germ line. It has been introduced into a laboratory mouse strain, and, by recombination, inserted into a Robertsonian metacentric chromosome for easier handling and identification.
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