High mobility group proteins (HMGs) are a set of chromatin related proteins named according to their high electrophoretic mobility on SDS-PAGE. The HMG protein family is subdivided into three subfamilies: the HMGB subfamily, the HMGA subfamily and the HMGN subfamily. Each of the subfamilies has a characteristic functional motif and these motifs are the main sites of interaction between the HMG proteins and the DNA or chromatin targets. The HMG proteins have many functions and all of them work as “architectural” factors by either distorting, bending or modifying the structure of DNA complexed with transcription factors or with histones.