The house mouse, Mus musculus, harbours a variable cluster of long-range repeats in chromosome 1. As shown in previous studies, some high-copy clusters such as the MUT cluster are cytogenetically apparent as a homogeneously staining region (HSR) and are associated with a distortion of the Mendelian recovery ratio when transmitted by heterozygous females. The effect is caused by a decreased viability of +/+ embryos. It is compensated by maternal or paternal MUT. In this study, a deletion derivative of MUT, MUTdel, shows normal transmission ratios and no compensating capability. In this respect, MUTdel behaves like a wild-type cluster. Hence, both properties--transmission ratio distortion and compensating capability--map to the deleted region. The deletion comprises three-quarters of the MUT HSR and does not extend to the nearest markers adjacent to the HSR.