Partial trisomy distal 4q (denoted 4q1) is a human chromosomal disorder caused by duplication of the distal end of the long arm of chromosome 4 (Chr4). This disorder manifests typical phenotypes, including craniofacial, renal, heart and thumb developmental defects. Although these clinical features are likely caused by a dosage imbalance in the gene network involving the trisomic region, the causative gene or genes and the molecular bases are largely unknown. Here, we report mouse

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... nduced mutation 4 (Rim4) as a model animal of 4q1. The Rim4 genome contains an insertion of a 6.5 Mb fragment from mouse chromosome 8 into chromosome 6. This insertion fragment contains 17 genes, including Hand2, that encode the basic helix-loop-helix transcription factor and is syntenic to the distal end of human Chr4, 4q32.3 to 4q34.1, which is responsible for 4q1. A comparison of phenotypes between patients with Rim4 and 4q1 revealed that Rim4 shows direct parallels with many phenotypes of 4q1 such as craniofacial, heart, cervical vertebra and limb deformities. Rebalancing the gene dosage by a genetic cross with Hand2 knockout mice ameliorated symptoms of the heart and limb deformities of Rim4. Conversely, an increase in copy number of Hand2 in wild-type mice recaptures the heart and limb deformities of Rim4. Our results collectively demonstrate that overdosage of Hand2 is a major cause for at least the limb and heart phenotypes of 4q1 and that mouse Rim4 provides a unique animal model for understanding the molecular bases underlying the complex phenotypes of 4q1.