The function of DNAJC9 (DnaJ heat shock protein family (Hsp40) member C9, Ensembl gene identifier ENSG00000213551) is as follows. Acts as a dual histone chaperone and heat shock co-chaperone (PubMed:33857403). As a histone chaperone, forms a co-chaperone complex with MCM2 and histone H3-H4 heterodimers; and may thereby assist MCM2 in histone H3-H4 heterodimer recognition and facilitate the assembly of histones into nucleosomes (PubMed:33857403). May also act as a histone co-chaperone together with TONSL (PubMed:33857403). May recruit histone chaperones ASF1A, NASP and SPT2 to histone H3-H4 heterodimers (PubMed:33857403). Also plays a role as co-chaperone of the HSP70 family of molecular chaperone proteins, such as HSPA1A, HSPA1B and HSPA8 (PubMed:17182002, PubMed:33857403). As a co-chaperone, may play a role in the recruitment of HSP70-type molecular chaperone machinery to histone H3-H4 substrates, thereby maintaining the histone structural integrity (PubMed:33857403). Exhibits activity to assemble histones onto DNA in vitro (PubMed:33857403). {ECO:0000269|PubMed:17182002, ECO:0000269|PubMed:33857403}.