In a recent review of chromosome 21 content, 552 genes were identified in the long arm of the chromosome (21q) [71], including 161 protein-coding genes cataloged in the Reference Sequence database of the National Center for Biotechnology Information (NCBI). The remaining 391 gene models are referred to as novel genes or non-cataloged genes, which could be protein-coding genes or functional RNA genes. Considering that the genetic basis of DS is the presence of three copies of chromosome 21, the first and most commonly accepted hypothesis for DS phenotype is that the genes in triplicate are overexpressed and, thus, the dosage imbalance of genes on chromosome 21 is responsible for the molecular dysfunctions in DS [72]. Among the genes present in chromosome 21, may be highlighted some described in the literature with overexpression associated with phenotypes of DS, most influencing the structure or function of the central nervous system (Table 1). Location of these genes on chromosome 21 is presented in Figure 1.