High-mobility group protein B3 is a protein that in humans is encoded by the HMGB3 gene. HMGB3 belongs to the higHuman, Mouseobility group (HMG) protein superfamily. Like HMG1 and HMG2, HMGB3 contains DNA-binding HMG box domains and is classified into the HMG box subfamily. Members of the HMG box subfamily are thought to play a fundamental role in DNA replication, nucleosome assembly and transcription.