Small supernumerary marker chromosomes (sSMC) in humans

Small supernumerary marker chromosomes (sSMC), defined as additional centric chromosome fragments too small to be identified or characterized unambiguously by banding cytogenetics alone, are present in 0.043% of newborn children. Several attempts have been made to correlate certain sSMC with a specific clinical picture, resulting in the description of several syndromes such as the i(18p)-, der(22)-, i(12p)- (Pallister Killian syndrome) and inv dup(22)- (cat-eye) syndromes. However, most of the remaining sSMC including minute-, ring-, inverted-duplication- as well as complex-rearranged chromosomes, have not yet been correlated with clinical syndromes, mostly due to problems in their comprehensive characterization. Here we present an overview of sSMC, including the first attempt to address problems of nomenclature and their modes of formation, problems connected with mosaicism plus familial occurrence. The review also discusses the frequency of sSMC in prenatal, postnatal, and clinical cases, their chromosomal origin and their association with uniparental disomy. A short review of the up-to-date approaches available for sSMC characterization is included. Clinically relevant correlations concerning the presence of a specific sSMC and its phenotypic consequences should become available soon.