A cluster of long-range repeats (LRRs) with a repeat size of roughly 100 kb is part of band D of chromosome 1 of the house mouse, Mus musculus. The cluster is cytogenetically polymorphic: it is either C-band negative or C-band positive. Our results show that the differential staining behavior depends on the LRR copy number and not on differences in DNA composition. There is a threshold between 105 and 175 LRR copies per haploid genome; clusters with lower copy numbers stain C-band negative, whereas those with higher copy numbers are C-band positive. Above this threshold, the size of the C-band is linearly correlated with the LRR copy number. The results imply that sequences capable of forming heterochromatin may be dispersed throughout the genome but are not recognized as such by cytogenetic techniques, unless they reach the threshold amount and concentration.