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Small supernumerary marker chromosomes sSMC originating from chromosome 10 are rare. Only seven cases have been documented, and among those three cases were diagnosed prenatally. We reported on another prenatal diagnosis of a de novo mosaic sSMC in an apparently normal female fetus whose mother had conceived with assisted reproductive technology ART procedures. Gbanding analysis of amniotic cells was performed. Spectral karyotyping SKY and fluorescence in situ hybridization FISH studies with chromosome 10specific alphoid satellite DNA probe were used to identify the chromosome 10 origin of the sSMC. Further FISH study with telomeric sequence probes showed that the sSMC lacked a hybridization signal, suggesting that the marker could be a ring chromosome. FISH studies using BAC clone probes specific for the regions within 10p11.2, 10q11.1, and 10q11.2 showed that the short arm breakpoint was located between 29.8 and 30.7 Mb from the 10p telomere, and that the long arm breakpoint was located less than 43.6 Mb from the 10p telomere. The karyotype of the fetus was 47,XX,mar. ish der10SKY CEP 10, CTD2130I7, RP1189J23−46,XX. Oligonucleotide microarraybased copy number variations CNV analysis was also performed and showed a 6.7 Mb duplication from 10p11.2 to 10q11.2 36.2–42.9 Mb with Affymetrix SNParray 6.0 genotype: arr cgh. 10p11.2q11.2CN519687 → CN541524 X 3. At the 1year followup, the baby did not have any findings of the trisomy 10p syndrome. This observation provided further credence to the concept that additional chromosome material of proximal 10p11.2 may not contribute to the trisomy 10p syndrome phenotype. © 2009 WileyLiss, Inc.