Dissecting the precise gene expression consequences of chromosome 21 trisomy is challenging because background genetic variation complicates comparisons between patients and healthy controls. Letourneau et al. used high-throughput RNA sequencing to characterize differences in gene expression in fetal skin fibroblasts from a human twin pair that were discordant for chromosome 21 trisomy but that were otherwise isogenic. The extra copy of chromosome 21 led to large domains of altered gene expression genome wide. These regions correlated with known domains that are associated with the nuclear lamina and DNA replication timing, and were characterized by differences in the activating chromatin mark H3K4me3. Thus, the trisomy has widespread consequences on the chromatin environment throughout the genome.