Small Supernumerary Ring Chromosomes

Small supernumerary ring chromosomes (sSRCs) are a specific subgroup of ring chromosomes (RCs) normally present in addition to an otherwise normal karyotype. Besides, they are smaller or the same size as a chromosome 20 of the same metaphase-spread. Accordingly, sSRCs at the same time fall into the overall group of small supernumerary marker chromosomes (sSMCs). sSRCs provide in most cases a gain of copy numbers of genomic regions adjacent to a centromere, while RCs predominantly lead to loss of copy numbers at the subtelomeric parts of a chromosome. Here, peculiarities of sSRCs are presented and discussed. It is concluded that they share most clinical features with sSMCs rather than with RCs.