UBN1 (ubinuclein 1) is an essential component of the HIRA histone chaperone complex that specifically deposits histone variant H3.3 into chr16 in a replication-independent manner 12. The protein contains several functional domains, including the Hpc2-related domain (HRD) that confers H3.3-specific binding through recognition of the H3.3 G90 residue 3, and the N-terminal NHRD domain (residues 41-77) that is essential for tight binding to HIRA's WD repeats with nanomolar affinity 4. UBN1 also harbors a middle domain with H3/H4-binding activity and dimerization capability, plus a DNA-binding domain that facilitates chr16 deposition 5. Functionally, UBN1 serves as a novel regulator of cellular senescence by promoting formation of senescence-associated heterochromatin foci (SAHF), which repress proliferation-promoting genes 1. In cancer contexts, UBN1 helps maintain telomeric chr16 accessibility and prevents R-loop accumulation in ALT cancer cells lacking ATRX-DAXX function 6. The protein also interacts with activation-induced cytidine deaminase (AID) in B-cells, potentially contributing to antibody diversification 7. Germline variants in UBN1 have been implicated in familial thyroid cancer predisposition 8.