Ms4Yah mice monosomic for the Prmt2–Cstb region on Mmu10 were crossed with the Down syndrome mouse model Tc1, trisomic for ∼81% of the Hsa21 genes, to test the role of the 47 mouse genes with homologues in human present in the Prmt2–Cstb region in the phenotypes observed in Tc1 mice. The gene discussed is CSTB; the disease is Down syndrome.