Down’s syndrome, also known as trisomy 21, is one of the most common genetic diseases. Researchers have recently analyzed the proteins of individuals with trisomy 21 for the first time: the goal was to improve our understanding of how a supernumerary copy of chromosome 21 could affect human development. The research shows that trisomy 21, far from only affecting the proteins encoded by the chromosome 21 genes, also impacts on the proteins encoded by the genes located on the other chromosomes.