Identification of supernumerary marker chromosomes derived from chromosomes 5, 6, 19, and 20 using FISH.

A large number of cases with supernumerary marker chromosomes (SMCs) should be compared to achieve a better delineation of karyotype-phenotype correlations. Here we present four phenotypically abnormal patients with autosomal marker chromosomes analysed by fluorescence in situ hybridisation using centromeric, telomeric, and unique sequence probes, as well as forward and reverse painting. We also report the first case, to the best of our knowledge, of an SMC derived from chromosome 5. Furthermore, a marker chromosome 20 in a patient with sex differentiation abnormalities, a double mar(6) in a boy with psychomotor retardation, and the association of r(19) with dup(21q21.2q22.12) are described. Although the mar(6) was very small, the presence of euchromatin was shown, suggesting that the partial trisomy of pericentric region derived sequences is implicated in the aetiology of the abnormal phenotypes.