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OBJECTIVE To report detail of a patient with infantile spasms whose cytogenetic analysis revealed mosaic monocentric and duplicated supernumerary marker chromosome (SMC) 15. SUBJECT AND METHODS The subject for this case was a 13-month-old girl with infantile spasms and delayed developmental milestones. Chromosomal analysis with G-band showed the presence of SMC in mosaic. Further investigations using in situ hybridization, methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA), microsatellite marker, and single nucleotide polymorphism (SNP) array analysis were performed. RESULTS Her karyotype was noted as mosaic 47,XX,+mar[26]/46,XX[4], ish der(15)(D15Z1+, SNRPN++, PML-) de novo. MS-MLPA analysis showed that the Prader-Willi syndrome/Angelman syndrome critical region is highly methylated, and microsatellite marker analysis proved that the 15q11.2 region of the patient comprises three kinds of alleles: one paternal and two maternal. SNP array analysis suggested an asymmetric structure of SMC(15) composed of 15q11-q13 recombination at breakpoint (BP) 4:BP5. CONCLUSIONS This is the first report of SMC(15) with monocentric and duplicated proximal 15q. The clinical presentations are quite similar to those of isodicentric chromosome 15 syndrome. The results of microsatellite and SNP array analysis suggest two possibilities regarding the timing of the mosaic SMC(15) formation. One possibility is that it occurred during maternal meiosis, and the other possibility is formation during a very early stage of embryo development that was initially trisomic of chromosome 15.