High-resolution cytogenetic studies of a normal individual with ring chromosome 19 indicate that, at the late-to-mid prophase level of band resolution, no apparent chromosomal material is missing, and that telomeric fusion/association, not deletion, is the cause of the ring chromosome formation. Sub-band analysis of the telomeric fusion shows thin chromatin filaments between the telomeres of some of the very elongated ring chromosomes, which cannot be resolved by metaphase chromosme analysis. The ring chromosome found in this individual shows evidence of the characteristic instability associated with ring chromosomes, including duplicated segments, double rings, and subsequent loss of the ring resulting in cells with monosomy 19. The lack of phenotypic effect and the unstable ring behavior, unlike previously reported patients with ring 19, support the formation of this ring by telomeric association.
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