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Ordinary lipoma is cytogenetically characterized by structural rearrangements, particularly translocations, of 12q13-15. By contrast, atypical lipomatous tumors exhibit supernumerary ring or giant marker chromosomes that are composed mainly of amplified material from the 12q13-15 chromosome segment. The present study describes the cytogenetic and molecular cytogenetic findings from a lipomatous tumor with minimal nuclear atypia that was identified in a 49-year-old female patient. Magnetic resonance imaging of the right shoulder revealed a 13-cm fatty mass in the subcutaneous layer that possessed only pencil-line septa. Contrast-enhanced fat-suppressed T1-weighted images demonstrated faint enhancement. A marginal excision was performed. Histologically, the tumor was composed of lobules that consisted of mature adipocytes separated by thin fibrous septa. There was minimal nuclear atypia in certain cells, and a small number of binucleated cells were also observed within the tumor. Immunohistochemically, the tumor cells did not reveal the expression of murine double-minute 2 (MDM2). Cytogenetic analysis revealed a complex karyotype with several numerical and structural alterations, including 12q rearrangements. Spectral karyotyping demonstrated a duplication of chromosome segment 12q13-15. Interphase fluorescence in situ hybridization analysis revealed no MDM2 gene amplification. The present case indicates that duplication of 12q may be associated with minimal nuclear atypia in a subset of lipomatous tumors. [ABSTRACT FROM AUTHOR]