Methods and compositions for reducing expression of genes on Chromosome 21 (“Chr 21”) by targeting an XIST transgene to the Dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A) gene or a Regulator of calcineurin 1 (RCAN1) gene, and cells and transgenic animals comprising an XIST transgene inserted into a DYRK1A or RCAN1 allele, e.g., cells and animals trisomic for human Chr 21 and mouse Chr 16.