A cloned DNA sequence that is specific for a germ line homogeneously staining region (HSR) on chromosome 1 of the mouse was found to be homologous to a single copy sequence in non-HSR mice. By in situ hybridization, the sequence in non-HSR mice was localized to approximately the same site as the insertion site of the HSR on chromosome 1 of HSR mice, indicating in situ amplification of the HSR.