In Saccharomyces cerevisiae, the HAP transcriptional complex is involved in the fermentation-respiration shift. This complex is composed of four subunits. Three subunits are necessary for DNA-binding, whereas the Hap4p subunit, glucose-repressed, contains the transcriptional activation domain. Hap4p is the key regulator of the complex activity in response to carbon sources in S. cerevisiae. To date, no HAP4 homologue has been identified, except in Kluyveromyces lactis. Examination of these two HAP4 sequences led to the identification of two very short conserved peptides also identified in other yeasts. In the yeast Hansenula polymorpha, two possible HAP4 homologues have been found. Their deduced amino acid sequences are similar to the ScHap4p and KlHap4p proteins only in the N-terminal 16-amino-acid basic motif. Since molecular genetic tools exist and complete genome sequence is known for this yeast, we expressed one of these putative HpHap4 proteins in S. cerevisiae and showed that this protein is able to restore the growth defect of the S. cerevisiae hap4-deleted strain. A set of experiments was performed to confirm the functional homology of this new gene with ScHAP4. The discovery of a Hap4-regulatory protein in H. polymorpha with only the N-terminal conserved domain of the S. cerevisiae protein indicates that this domain may play a crucial role during evolution.