The HMG-1 and-2 proteins have three structural domains. Two of these domains, termed A and B, have significant homology with each other, and the third comprises an acidic carboxy-terminal tail that can interact with histone H1 in vitro (reviewed in Refs 2, 3). Recent analysis of the RNA polymerase I transcription factor UBF by Tjian and co-workers identified a repeated region of 85 amino acids as a novel DNA-binding motif 4. On the basis of its homology with both the A and B domains of HMG-1, this repeat was temled the'HMG box'. In keeping with the terminology used for homeodomain proteins, we refer to this DNA-binding motif as the HMG domain and to the corresponding gene segment as the HMG box. Members of the HMG-1/2 family, which we classify as HMG domain proteins, can be divided into two subfamilies according t,: the number of HMG domains