The HMG proteins are subdivided into 3 superfamilies each containing a characteristic functional domain:
* HMGA – contains an AT-hook domain
** HMGA1
** HMGA2
* HMGB – contains a HMG-box domain
** HMGB1
** HMGB2
** HMGB3
** HMGB4
* HMGN – contains a nucleosomal binding domain
** HMGN1
** HMGN2
** HMGN3
** HMGN4
** HMGN5
Proteins containing any of these embedded in their sequence are known as HMG motif proteins. 
HMG-box proteins are found in a variety of eukaryotic organisms.
They were originally isolated from mammalian cells, and named according to their electrophoretic mobility in polyacrylamide gels.