ABSTRACT. A ring chromosome 13 or r(13) exhibits breakage and reunion at breakage points on the long and short arms of chromosome 13, with deletions of the chromosomal segments distal to the breakage points. The r(13) chromosome accounts for approximately 20% of ring chromosomes compatible with life. We describe a female patient with mental retardation, growth retardation, microcephaly, craniofacial dysmorphy, hearing impairment, and prolonged prothrombin time. Chromosomal analysis via GTG banding of peripheral blood lymphocy tes revealed a karyotype of 46,XX,r (13) (109,743,110,721)]. A cytogenetic study of peripheral blood revealed a rare chromosomal abnormality associated with different cell lines that included structural and numerical abnormalities of chromosome 13. This case, along with 14 previously reported cases, indicate that the smallest critical region for chromosome 13 microcephaly is 109,743,729-144,110,721.
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