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A ring 12 chromosome was found in a male child with minor phenotypic alterations. No obvious loss of chromosome material was detected. Since there is no other case of a ring 12 in the literature, it was not possible to determine whether the phenotype was due to (invisible) terminal deletions or to karyotypic variation. In lymphocyte cultures 9% of the cells had either no or two rings, but the patient's RBC had normal activities of the enzymes lactate dehydrogenase B and peptidase B, whose loci are located on the proximal portions of 12p and 12q, respectively. Dicentric rings were found in 37 cells, and all had two active centromeres, in contrast with the relatively frequent finding of latent centromeres in translocated dicentric autosomes. Two latent centromeres were found in one tricentric "rod-shaped" open ring 12, probably derived from a tetracentric ring. It is postulated that latent centromeres are rare in ring chromosomes because, if consistent suppression of centromeres in excess of one took place at each duplication, rings with four, eight, or more centromeres would be formed rather frequently, which is not the case.