Multiple “marker” chromosomes: A novel cytogenetic finding in a patient with mental retardation and congenital anomalies

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Abstract
A patient with mental retardation and clinical manifestations suggestive of Noonan syndrome was found to have in her peripheral lymphocytes multiple small accessory marker chromosomes, varying in number from one to five per cell and in size from about half the size of the q arm of a G group chromosome to less than a centromere. Occasionally, in the more elongated markers, a G‐positive or a C‐positive band could be identified, or the marker had the appearance of a ring. The origin and significance of these marker chromosomes are discussed.