Small supernumerary marker chromosomes and uniparental disomy have a story to tell.

Small supernumerary maker chromosomes (sSMC) and uniparental disomy (UPD) are rare, and a combination of both is rarely encountered. Accordingly, only 46 sSMC cases UPD have been reported. Despite of its rareness, UPD has to be considered, especially in prenatal cases with sSMC. Here, the authors reviewed all sSMC cases with UPD (sSMC(U+)) and compared them to sSMC without UPD (sSMC(U-)), which resulted in the following correlations: 1) every sSMC, irrespective of its chromosomal origin, may be principally connected with UPD; 2) mixed hetero- and iso-UPD (hUPD/iUPD) can be observed most often in sSMC(U+) cases followed by complete iUPD, complete hUPD, and segmental iUPD; 3) UPD of chromosomes 6, 7, 14, 15, 16, and 20 is most often reported in sSMC(U+); 4) maternal UPD was approximately nine times more frequent than paternal UPD; 5) if mosaic with a normal cell line, acrocentric-derived sSMC had a three times higher chance of occurrence than the corresponding nonmosaic sSMC cases; 6) UPD in connection with a parentally inherited sSMC is, if existent at all, a rare event; and 7) the gender type and shape of sSMC had no effect on UPD formation. Overall, sSMC(U+) cases may have a story to tell about chromosome number control mechanisms in early embryogenesis.