Abstract: HMG1 and 2 (high mobility group proteins 1 and 2; renamed HMGB1 and 2) contain two DNA-binding HMG-box domains (A and B) and a long acidic C-terminal domain. They bind DNA without sequence specificity, but have a high affinity for bent or distorted DNA, and bend linear DNA. The individual A and B boxes (which, although broadly similar, show both structural and functional differences) exhibit many of the structure-specific properties of the whole protein. The acidic tail modulates the affinity of the tandem HMG boxes in HMG1 and 2 for a variety of DNA targets, including four-way junctions, but not distorted DNA minicircles, to which the proteins bind with very high affinity. HMG1 and 2 appear to play important architectural roles in the assembly of nucleoprotein complexes in a variety of biological processes, for example V(D)J recombination, the initiation of transcription, and DNA repair.