The pseudoautosomal region of the Movl5 mouse strain is marked by a Moloney murine leukemia provirus. The sequences flanking the Movl5 provirus were molecularly cloned and shown to consist of a tandemly repeated sequence of 31 nucleotides. Copy number variation of this repeat most likely accounts lor the polymorphism in the mouse pseudoautosomal region detected with a probe from the flanking sequences. In situhybridization to metaphase chromosomes showed heavy labeling of the pairing region of the X and Y chromosomes. The repetitive sequence was also found at the subtelomeric region of three autosomes. A similar level of amplification as the one seen on the sex chromosomes seems to be present on chromosomes 9 and 13. Lower copy numbers appear to be present on chromosome 4.