Characterization of prenatally assessed de novo small supernumerary marker chromosomes by molecular cytogenetics.

Small supernumerary marker chromosomes (sSMC) are structurally abnormal chromosomes that cannot be identified or characterized unambiguously by conventional banding cytogenetics alone, and they are generally equal in size or smaller than a chromosome 20 of the same metaphase spread. sSMC are reported in 0.043% of newborn infants and 0.075% of prenatal cases. Molecular cytogenetics is necessary to characterize the origin of an sSMC, and many highly sophisticated approaches are available throughout the literature for their comprehensive description. However, because in a prenatal diagnostic laboratory such techniques are not available, I suggest here a straightforward scheme to characterize at least the sSMC's chromosomal origin as quickly as possible. Based on this scheme, it is possible to compare the actual present case with similar cases from the literature, which are summarized on http://www.med.uni-jena.de/fish/sSMC/00START.htm./ For a more wide-ranging sSMC characterization, a specialized laboratory should be contacted, e.g., my laboratory.