The CCAAT-binding complex is an evolutionarily conserved heterotrimeric transcriptional factor in eukaryotes including fungi, plants and mammals. In Aspergillus nidulans, the corresponding complex was designated CBC (CCAAT-binding complex). CBC consists of three subunits HapB, HapC and HapE. Interestingly, only HapC contains three cysteine residues. To study the role of these cysteines on the stability of CBC in A. nidulans, the hapC gene was mutated, in which all three cysteines were exchanged by serines (hapC3CS) and by alanines (hapC3CA), and fused to egfp. Furthermore, the redox regulation of HapC under oxidative stress conditions was analyzed in vivo. The analysis of a HapC-EGFP fusion protein indicated that the regulation of HapC is dependent on the redox status of the cell.