The function of DNAJB9 (DnaJ heat shock protein family (Hsp40) member B9, Ensembl gene identifier ENSG00000128590) is as follows. Co-chaperone for Hsp70 protein HSPA5/BiP that acts as a key repressor of the ERN1/IRE1-mediated unfolded protein response (UPR) (By similarity). J domain-containing co-chaperones stimulate the ATPase activity of Hsp70 proteins and are required for efficient substrate recognition by Hsp70 proteins (PubMed:18400946). In the unstressed endoplasmic reticulum, interacts with the luminal region of ERN1/IRE1 and selectively recruits HSPA5/BiP: HSPA5/BiP disrupts the dimerization of the active ERN1/IRE1 luminal region, thereby inactivating ERN1/IRE1 (By similarity). Also involved in endoplasmic reticulum-associated degradation (ERAD) of misfolded proteins. Required for survival of B- cell progenitors and normal antibody production (By similarity). {ECO:0000250|UniProtKB:G3H0N9, ECO:0000250|UniProtKB:Q9QYI6, ECO:0000269|PubMed:18400946}.