High-mobility group protein B1 (HMGB1) is also known as high-mobility group protein 1 (HMG- 1) and amphoterin. HMGB1 is a non-histone nuclear protein with a dual function. In the nucleus, HMGB1 is a DNA binding protein that associates with chromatin and can bend DNA. Binds preferentially single-stranded DNA. Involved in V(D)J recombination by acting as a cofactor of the RAG complex. Acts by stimulating cleavage and RAG protein binding at the 23 bp spacer of conserved recombination signal sequences (RSS). Outside the cell, HMGB1 displays cytokine activity and can promote inflammation, serving as a mediator in models of shock and arthritis.