Normal human centromeres contain large tandem arrays of β£-satellite DNA of varying composition and complexity. However, a new class of mitotically stable marker chromosomes which contain neocentromeres formed from genomic regions previously devoid of centromere activity was described recently. These neocentromeres are fully functional yet lack the repeat sequences traditionally associated with normal centromere function. We report here a supernumerary marker chromosome derived from the short arm of chromosome 20 in a patient with manifestations of dup(20p) syndrome. Detailed cytogenetic, FISH, and polymorphic microsatellite analyses indicate the de novo formation of the marker chromosome during meiosis or early postzygotically, involving an initial chromosome breakage at 20p11.2, followed by an inverted duplication of the distal 20p segment due to rejoining of sister chromatids and the activation of a neocentromere within 20p12. This inv dup(20p) marker chromosome lacks detectable centromeric β£-satellite and pericentric satellite III sequences, or centromere protein CENP-B. Functional activity of the neocentromere is evidenced by its association with 5 different, functionally critical centromere proteins: CENP-A, CENP-C, CENP-E, CENP-F, and INCENP. Formation of a neocentromere on human chromosome 20 has not been reported previously and in this context represents a new mechanism for the origin of dup(20p) syndrome. Am. J. Med. Genet. 85:403-408, 1999.