We recently helped to complete the sequence of human chromosome 21 at a very high level of accuracy. Using this sequence we identified two novel genes, designated DSCR9 and DSCR10, in the so-called Down Syndrome Critical Region (DSCR) by computational gene prediction and subsequent cDNA cloning. Both DSCR9 and DSCR10 are expressed preferentially in testis and encode functionally unknown proteins with 149 and 87 amino acid residues, respectively. Zoo blot analysis suggested that both genes are exclusive to primate genomes such as chimpanzee, gorilla, orangutan, crab-eating monkey and African green monkey but are not present in other non-primate mammals including mouse, dog, cat, and chicken. Comparative genomic sequence analysis of DSCR9 and DSCR10 with the corresponding mouse syntenic region confirmed the lack of these genes in the mouse. These results strongly suggest that DSCR9 and DSCR10 have emerged as a new class of gene in the primate lineage during evolution.