Small supernumerary marker chromosomes (sSMC) can appear in a numerically normal ‘basic karyotype’, but also in a numerically abnormal one like a Turner syndrome karyotype (= sSMCT). Here we present 17 new cases with such a mos 45,X/46,X,+mar karyotype. Moreover we reviewed all 512 cytogenetically similar cases available from the literature and supply for the first time data on occurrence, shapes and subgroups of this rare cytogenetic entity. sSMCT are very rare in the common population (1:100,000) – however, they can be observed with a 45- and even 60-times higher frequency in infertile and (develop)mentally retarded patients, respectively. Even though sSMCT derive from one of the gonosomes in >99% of the cases, there are also exceptional reports on sSMCT derived from one of the autosomes. The majority of sSMCT(X) form ring chromosomes, while most sSMCT(Y) are inverted duplicated/isodicentric chromosomes. Although >500 sSMCT are reported, a detailed characterization of the chromosomal breakpoints is only given for a minority. Thus, more cases with detailed (molecular) cytogenetic marker chromosome characterization are needed to provide information on formation and effects of an sSMCT.

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