The defining feature of the Hsp40 chaperone family is a approximately 70-amino-acid-residue signature, termed the J domain, that is necessary for orchestrating interactions with its Hsp70 chaperone partner(s). J-domain proteins play important regulatory roles as co-chaperones, recruiting Hsp70 partners and accelerating the ATP-hydrolysis step of the chaperone cycle. Certain proteins could have acquired a J domain in order to present a specific substrate(s) to an Hsp70 partner and thus capitalize upon chaperone activities when carrying out cellular functions. J-domain proteins participate in complex biological processes, such as cell-cycle control by DNA tumor viruses, regulation of protein kinases and exocytosis.