Trisomy 21 with a Small Supernumerary Marker Chromosome Derived from Chromosomes 13/21 and 18

We describe a trisomy 21 with a small supernumerary marker chromosome (sSMC) derived from chromosomes 13/21 and 18 in which the karyotype was 48, XY, +der(13 or 21)t(13 or 21;18)(13 or 21pter→13q11 or 21q11.1::18p 11.21→18pter),+21. Of the 35 case reports in the literature for a karyotype 48, XN, +21,+mar, in only 12 was the origin of the sSMC determined by fluorescence in situ hybridization (FISH), and only one was a der(13 or 21) and none were derived from two chromosomes. The influence of the partial trisomy 18p on the clinical outcome was hard to determine, however, there are reports on clinically healthy subjects for partial trisomy 18p.