DNAJC9 is a dual-function J-domain co-chaperone that integrates heat shock protein machinery into histone management pathways 1. As a histone chaperone, DNAJC9 forms complexes with MCM2 and histone H3-H4 heterodimers, facilitating nucleosome assembly and recruiting additional histone chaperones including ASF1A, NASP, and SPT2 1. Mechanistically, DNAJC9 recruits HSP70-family chaperones via its J domain to fold histone substrates and resolve aberrant histone intermediates during replication- and transcription-coupled nucleosome assembly 1. DNAJC9 also promotes histone H3-H4 degradation, counteracted by the chaperone Asf1 to prevent excessive histone downregulation 2. Functionally, DNAJC9 maintains fidelity of histone supply chains by preventing CENP-A mislocalization to non-centromeric regions and associated chr10 instability 3. Beyond histones, DNAJC9 enhances hepatitis B virus cccDNA transcription by recruiting histone H3.3 and promoting active chr10 marks 4. Clinically, elevated DNAJC9 expression predicts worse survival in basal-like and luminal A breast cancers 5, and serves as a diagnostic biomarker for ocular adnexal B-cell lymphomas 6. DNAJC9 has also been associated with schizophrenia subtypes characterized by attention and executive function deficits 7.