HMGB1 binding to the RAG/DNA complex. HMGB1 binding was determined from the published cryo-electron microscopy and X-ray crystal structures of mouse RAG/RSS complexes.^12,13  Blue and green cartoons represent individual RAG1 monomers. (Left) HMGB1 Box A from the 23-RSS structure was superimposed over the density that likely corresponds to HMGB1 Box A at the 12-RSS, denoted by the asterisk.¹³  This falls directly opposite R401 of mouse RAG1, indicated in red. R401 contacts DNA in the 12-RSS spacer, close to the RSS nonamer. (Right) HMGB1 Box B binding is observed in the middle of the 23-RSS spacer, whereas an additional area of HMGB1 binding (Box A) lies opposite R401 (in red) at the 23-RSS. Data from PDB 6CG0.¹³