The homeobox domain-containing transcription factors play an important role in the growth, development, and secondary metabolism in fungi and other eukaryotes. In this study, we characterized the roles of the genes coding for homeobox-type proteins in the model organism Aspergillus nidulans. to examine their roles in A. nidulans, the deletion mutant strains for each gene coding for homeoboxtype protein were generated, and their phenotypes were examined. Phenotypic analyses revealed that two homeobox proteins, HbxA and HbxB, were required for conidia production. Deletion of hbxA caused abnormal conidiophore production, decreased the number of conidia in both light and dark conditions, and decreased the size of cleistothecia structures. Overexpressing hbxA enhanced the production of asexual spores and formation of conidiophore under the liquid submerged conditions. The hbxB deletion mutant strains exhibited decreased asexual spore production but increased cleistothecia production. The absence of hbxB decreased the trehalose content in asexual spores and increased their sensitivity against thermal and oxidative stresses. The ΔhbxA strains produced more sterigmatocystin, which was decreased in the ΔhbxB strain. Overall, our results show that HbxA and HbxB play crucial roles in the differentiation and secondary metabolism of the fungus A. nidulans.Aspergillus nidulans is a model filamentous fungus commonly used for the understanding of the genetics and molecular biology of fungal development and secondary metabolism 1-3 . The reproductive cycle of A. nidulans can be divided into two phases as asexual and sexual 4,5 . This fungus forms specialized developmental structures, which were used to designate the name of this fungus. Under the conditions that induce asexual reproduction, A. nidulans forms typical aspergillum-like structures called conidiophores, which contain a vesicle each, metulae, phialides, and conidia 6-8 . During the sexual cycle, A. nidulans (anamorph: Emericella nidulans) produces the nest-like sexual fruiting body called cleistothecium 9,10 . The formation of asexual or sexual structures involves sophisticated molecular events that are regulated by a variety of elements, especially transcription factors 4,5 .Transcription factors have sequence-specific DNA-binding motifs and control the transcription levels of target genes 11 . To date, approximately 80 transcription factor families have been described in the fungal genome 12 . These transcription factors positively or negatively control gene expression during fungal development 5 . Several developmental activators (e.g., FlbB, FlbC, FlbD, and FlbE) or repressors (e.g., SfgA and NsdD) are involved in the activation of BrlA, an essential transcription factor initiating conidia formation and the asexual cycle 6,13,14 . BrlA contains a C 2 H 2 zinc finger DNA-binding domain and controls the transcription of abaA, a key gene involved in the mid phase of conidiogenesis [15][16][17] . AbaA mainly governs the differentiation of phialides and activates the sp...