Small supernumerary marker chromosomes (sSMCs) are small supernumerary aberrant chromosomes that are generally equal in size or smaller than a chromosome 20, and cannot be identified or characterized by conventional cytogenetic banding techniques [1–3]. sSMCs can appear in 0.044% of newborn infants and in 0.075% of prenatal cases [1,3,4]. About 70% of sSMCs are caused by a de novo event [5], about 70% of sSMCs are originated from acrocentric chromosomes [1,6], and about 70% of de novo sSMCs are associated with no phenotypic effects [4]. Prenatal diagnosis of sSMCs gives rise to difficulties in genetic counseling, and identification of the nature of the aberrant chromosome requires molecular cytogenetic technologies [4,7–10]. We present our experience of the prenatal diagnosis and molecular cytogenetic characterization of an sSMC derived from chromosome 21 using fluorescence in situ hybridization (FISH) and multiplex ligation-dependent probe amplification (MLPA). A 36-year-old woman, gravida 2, para 1, underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed an sSMC, which was C-band positive and nucleolar organizing region-stain positive. The parental karyotypes were normal. The karyotype was 47,XX,+mar (Figure 1). The sSMC hybridized with a centromere 13/21-specific α-satellite DNA probe (D13Z1/D21Z1) (cep13/21) (Cytocell, Adderbury, Oxfordshire, UK) (Figure 2). MLPA was used to determine the origin of the sSMC using a SALSA MLPA P181 centromere kit (MRC-Holland, Amsterdam, the Netherlands) (Figure 3). The results of MLPA indicated a duplication of the 21q11.2 segment containing the STCH and SAMSN1 genes. FISH determination of the duplications of the STCH and SAMSN1 genes was performed using the bacterial artificial chromosome clone probes RP11-138O15 and RP11-392H8 at 21q11.2. FISH determination of 21q21.1 involvement utilized the 21q21.1-specific bacterial artificial chromosome clone probes RP11-89M24 and RP11-109H14. FISH revealed STCH and SAMSN1 gene duplications in the fetus with sSMC(21) (Figure 4). No RP11-89M24or PRENATAL DIAGNOSIS AND MOLECULAR CYTOGENETIC CHARACTERIZATION OF A SMALL SUPERNUMERARY MARKER CHROMOSOME DERIVED FROM CHROMOSOME 21