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We report the case of a patient with a clinical phenotype consistent with Down Syndrome (DS) who has a novel karyotypic abnormality. Karyotypic analyses were performed to investigate the cause of two spontaneous abortions. A balanced translocation between chromosomes 4 and 21 was identified, along with an additional abnormal chromosome 21. We performed high-resolution banding, comparative genomic hybridization (CGH), and FISH studies in both the patient and her mother to define the abnormality and determine its origin. CGH revealed a gain in copy number on the long arm of chromosome 4, spanning at least 24.4 Mb, and a gain in copy number on the long arm of chromosome 21, spanning at least 16.2 Mb. FISH analysis using a chromosome 21 centromere probe and chromosome 4 long arm telomere (4pter) probe confirmed the origin of the marker chromosome. It has been confirmed by the State Key Laboratory of Medical Genetics of China that this is the first reported instance of the karyotype 47,XX,t(4;21)(q31.3;q11.2),+der(21)t(4;21)mat reported in the world.