Updated project metadata. The Dp1Tyb mouse model for Down syndrome contains a duplication of 23Mb of mouse chromosome 16 (Mmu16) that is orthologous to human chromosome 21 (Hsa21). This region contains 145 coding genes, and thus these genes are present in 3 copies. Dp1Tyb mice display congenital heart defects, similar to the ones seen in people with Down syndrome. These defects include ventricular and atrio-ventricular septal defects and are seen at embryonic day 14.5 (E14.5) of gestation. One of the 145 genes present in 3 copies in Dp1Tyb mice codes for a kinase called DYRK1A. We found that by crossing Dp1Tyb mice with mice carrying a heterozygous Dyrk1a loss of function allele, thereby reducing the dosage of the Dyrk1a gene from 3 to 2 copies, we rescue the congenital heart defects. Thus 3 copies of Dyrk1a are required to cause heart defects, and, presumably, increased DYRK1A protein is required for the heart defects. We compared the phosphoproteome in Dp1Tyb versus Dp1TybDyrk1a+/+/- embryonic hearts in order to discover alterations in phosphoproteins that could pinpoint molecular mechanisms that give rise to the congenital heart defects. The two strains (Dp1Tyb and Dp1TybDyrk1a+/+/-) differ only in the copy number of Dyrk1a (3 v 2) and thus differences in phosphoproteins would include both direct and indirect targets of DYRK1A activity.