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 correla tions. Here we present: four phenotypically abnormal patients with autosoma l marker chromosomes analysed by fluorescence in situ hybridisation using c entromeric, telomeric, and unique sequence probes, as well as forward and r everse painting. We also report the first case, to the best of our knowledg e, 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(21q 21.2q22.12) are described. Although the mar(6) was very small, the presence of euchromatin was shown, suggesting that the partial trisomy of pericentr ic region derived sequences is implicated in the aetiology of the abnormal phenotypes.