The smallest of the human chromosomes, 21 makes up about 1.5% of the human genome. Chromosome 21 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. DnaJC28 (DnaJ homolog subfamily C member 28), also known as C21orf55, C21orf78 or MGC27620 is a 388 amino acid protein that is expressed in the fetal and adult brain, testis, uterus, spleen and liver. It has been suggested that DnaJC28 may play a role in protein folding or as a chaperone. The DnaJC28 gene product has been provisionally designated DnaJC28 pending further characterization.