With the presence of the J domain defining a protein as a member, the DnaJ family has evolved with diverse cellular localization and functions and is one of the largest chaperone families. DnaJ heat-shock-induced proteins are derived from the bacterium Escherichia coli and are controlled by the htpR regulatory protein. DnaJ proteins play a critical role in the HSP 70 chaperone machine by interacting with HSP 70 to stimulate ATP hydrolysis. Members of this family contain cysteine-rich regions composed of zinc fingers that form a peptide-binding domain responsible for chaperone function. DnaJ proteins are important mediators of proteolysis and are involved in the regulation of protein degradation, exocytosis and endocytosis. DNAJC21 (DnaJ homolog subfamily C member 21), also known as DNAJA5 or JJJ1, is a 531 amino acid protein that contains two C2H2-type zinc fingers and one J domain. Expressed in placenta, pancreas, kidney and brain, DNAJC21 may be a co-chaperone for HSP 70.