The function of DNAJB1 (DnaJ heat shock protein family (Hsp40) member B1, ENSG00000132002) is as follows. Interacts with HSP70 and can stimulate its ATPase activity. Acts also with TTC1 as a chaperone adapter that regulates HSP70- dependent folding process by interacting with the HSP70 amino terminal region (PubMed:14503850). Stimulates the association between HSC70 and HIP. Negatively regulates heat shock-induced HSF1 transcriptional activity during the attenuation and recovery phase period of the heat shock response (PubMed:9499401). Stimulates ATP hydrolysis and the folding of unfolded proteins mediated by HSPA1A/B (in vitro) (PubMed:24318877). {ECO:0000269|PubMed:14503850, ECO:0000269|PubMed:24318877, ECO:0000269|PubMed:9499401}.