Histone chaperone that plays a role in the nuclear import of H2A-H2B and nucleosome assembly (PubMed:20002496, PubMed:26841755). Participates also in several important DNA repair mechanisms: greatly enhances ERCC6-mediated chromatin remodeling which is essential for transcription-coupled nucleotide excision DNA repair (PubMed:28369616). Stimulates also homologous recombination (HR) by RAD51 and RAD54 which is essential in mitotic DNA double strand break (DSB) repair (PubMed:24798879). Plays a key role in the regulation of embryonic neurogenesis (By similarity). Promotes the proliferation of neural progenitors and inhibits neuronal differentiation during cortical development (By similarity). Regulates neurogenesis via the modulation of RASSF10; regulates RASSF10 expression by promoting SETD1A-mediated H3K4 methylation at the RASSF10 promoter (By similarity). (Microbial infection) Positively regulates Epstein-Barr virus reactivation in epithelial cells through the induction of viral BZLF1 expression. (Microbial infection) Together with human herpesvirus 8 protein LANA1, assists the proper assembly of the nucleosome on the replicated viral DNA. Homodimer (PubMed:26841755). The dimer binds strongly and sequentially to single and double H2A-H2B heterodimers (PubMed:26841755). Interacts with ERCC6; this interaction increases ERCC6 processivity (PubMed:28369616). Interacts with RAD54 (PubMed:24798879). Interacts with SETD1A (By similarity). (Microbial infection) Interacts with human herpesvirus 8 protein LANA1 (via N-terminus); this interaction is required for LANA1-dependent DNA replication. (Microbial infection) Interacts with hepatitis virus protein NS5A (via C-terminus); this interaction sequesters NAP1L1 in the cytoplasm, blocking its nuclear translocation. (Microbial infection) Interacts with Chikungunya virus non-structural protein 3 (via C-terminus). Identified by mass spectrometry in melanosome fractions from stage I to stage IV. Ubiquitously expressed. The acidic domains are probably involved in the interaction with histones. Monoglycylated on glutamate residues. Cannot be polyglycylated due to the absence of functional TTLL10 in human (By similarity). Polyglutamylated by TTLL4 on glutamate residues, resulting in polyglutamate chains on the gamma-carboxyl group. Both polyglutamylation and monoglycylation modifications can coexist on the same protein on adjacent residues, and lowering polyglycylation levels increases polyglutamylation, and reciprocally. Belongs to the nucleosome assembly protein (NAP) family.