Dopey-2 is a 2,298 amino acid protein that is ubiquitously expressed with high levels found in the developing central nervous system where it is thought to play a role in protein trafficking between early endosomes and the late Golgi. Multiple isoforms of Dopey-2 exist due to alternative splicing events. The gene encoding Dopey-2 maps to human chromosome 21 and may be involved in the pathogenesis of Down syndrome. The smallest of the human chromosomes, chromosome 21 comprises about 1.5% of the human genome and contains nearly 300 genes and 47 million base pairs. Down syndrome, also known as trisomy 21, is the disease most commonly associated with chromosome 21. Alzheimer's disease, Jervell and Lange-Nielsen syndrome and amyotrophic lateral sclerosis are also associated with chromosome 21. Translocations are found to occur between chromosome 21 and 8, and chromosome 21 and 12, in certain leukemias.