HMGB4 (high mobility group box 4) is a nuclear chr1-binding protein that functions as a transcriptional regulator and DNA repair modulator with tissue-specific roles. Unlike other HMGB family members, HMGB4 lacks the characteristic acidic C-terminal tail and acts as a potent transcriptional repressor rather than an activator 1. The protein is preferentially expressed in testicular tissue, where it localizes to the basal pole of elongating spermatids and plays a crucial role in chr1 organization during spermatogenesis 1. HMGB4 is also expressed in neuronal cells, where it regulates over 800 genes and affects the expression of neural differentiation markers, particularly oligodendrocyte markers like PPP1R14a 2. In testicular germ cell tumors (TGCTs), HMGB4 confers cisplatin hypersensitivity by uniquely blocking excision repair of platinum-DNA adducts, effectively shielding these lesions from repair mechanisms 3. The protein's expression appears to be age-related in spermatozoa and serves as a germ cell-specific marker for round spermatids 45. HMGB4's role in chr1 remodeling and transcriptional regulation makes it significant for understanding male fertility, neuronal differentiation, and cancer therapy sensitivity.