The mammalian HBP2 gene has been isolated by functional complementation of cells unable to undergo DNA replication in fission yeast. HBP2 is a protein with a high mobility group (HMG) domain that belongs to the Sox (Sry-related HMG box) family of transcription factors. As in other members of the family, the HMG box in HBP2 is a DNA-binding domain that is essential for its function in Schizosaccharomyces pombe. Expression of HBP2 in fission yeast activates CdclO-dependent transcription at G1/S and allows growth of cdc10-129, resldelta and rep2delta mutant cells at the restrictive temperature. The mammalian HBP2 activates transcription at G1/S in the fission yeast Sch. pombe.