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Background  Small supernumerary marker chromosomes (sSMC) occur in 0.072% of unselected cases of prenatal diagnoses, and their molecular cytogenetic characterization is required to establish a reliable karyotype-phenotype correlation. A small group of sSMC are C-band-negative and devoid of alpha-satellite DNA. We report the molecular cytogenetic characterization of a  de novo  analphoid sSMC derived from 18q22.1→qter in cultured amniocytes.    Results  We identified an analphoid sSMC in cultured amniocytes during a prenatal diagnosis performed because of advanced maternal age. GTG-banding revealed an sSMC in all metaphases. FISH experiments with a probe specific for the chromosome 18 centromere, and C-banding revealed neither alphoid sequences nor C-banding-positive satellite DNA thereby suggesting the presence of a neocentromere. To characterize the marker in greater detail, we carried out additional FISH experiments with a set of appropriate BAC clones. The pattern of the FISH signals indicated a symmetrical organization of the marker, the breakpoint likely representing the centromere of an inverted duplicated chromosome that results in tetrasomy of 18q22.1→qter. The karyotype after molecular cytogenetic investigations was interpreted as follows:  47,XY,+inv dup(18)(qter→q22.1::q22.1→neo→qter)    Conclusion  Our case is the first report, in the prenatal diagnosis setting, of a  de novo  analphoid marker chromosome originating from the long arm of chromosome 18, and the second report of a neocentromere formation at 18q22.1.

Molecular characterization of an analphoid supernumerary marker chromosome derived from 18q22.1→qter in prenatal diagnosis: a case report