HMG-box containing protein 1 (HBP1) is a member of the high mobility group (HMG) of chromosomal proteins. It is a sequence-specific HMG transcription factor. Several features of HBP1 suggest an intriguing role as a transcriptional and cell cycle regulator in differentiated cells and it may represent a unique transcriptional repressor with a role in initiation and establishment of cell cycle arrest during differentiation.