Copy-number variation in the human genome can be disease-causing or phenotypically neutral. This type of genetic rearrangement associated with human chromosome 21 (Hsa21) underlies partial Monosomy 21 and Trisomy 21. Mental retardation is a major clinical manifestation of partial Monosomy 21. To model this human chromosomal deletion disorder, we have generated novel mouse mutants carrying heterozygous deletions of the 2.3- and 1.1-Mb segments on mouse chromosome 10 (Mmu10) and Mmu17, respectively, which are orthologous to the regions on human 21q22.3, using Cre/loxP-mediated chromosome engineering. Alterations of the transcriptional levels of genes within the deleted intervals reflect gene-dosage effects in the mutant mice. The analysis of cognitive behaviors shows that the mutant mice carrying the deletion on either Mmu10 or Mmu17 are impaired in learning and memory. Therefore, these mutants represent mouse models for Monosomy 21-associated mental retardation, which can serve as a powerful tool to study the molecular mechanism underlying the clinical phenotype and should facilitate efforts to identify the haploinsufficient causative genes.