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Trisomy for chromosome 21 (Chr 21) has profound effects on development that result in a constellation of phenotypes known as Down syndrome (DS). Distinctive craniofacial manifestations are among the few features common to all individuals with DS. The characteristic face of a person with DS results primarily from maldevelopment of the underlying craniofacial skeleton. The Ts65Dn mouse, which has segmental trisomy 16, producing dosage imbalance for about half the genes found on human Chr 21, exhibits specific skeletal malformations corresponding directly to the craniofacial dysmorphogenesis in DS. Here we demonstrate that Ts1Cje mice, which are at dosage imbalance for about 3/4 of the genes triplicated in Ts65Dn, demonstrate a very similar pattern of anomalies in the craniofacial skeleton. However, one characteristic of Ts65Dn mice, a broadening of the cranial vault contributing to brachycephaly, is …