8 pages, 3 tables, 2 figures.-- PMID: 16436696 [PubMed].-- PMCID: PMC1366924. Two erg3 genes encoding C-5 sterol desaturase enzymes (Erg3A and Erg3B) in Aspergillus fumigatus were characterized with respect to their nucleotide sequences and null mutant phenotypes. Targeted disruption of the erg3A and erg3B genes and a double gene knockout, erg3A– erg3B–, showed that they are not essential for A. fumigatus viability. Mutant phenotypes clearly showed that Erg3B is a C-5 sterol desaturase, but no apparent role for Erg3A in A. fumigatus ergosterol biosynthesis was found. Susceptibility to amphotericin B, itraconazole, fluconazole, voriconazole, and ketoconazole was not altered in isolates in which erg3A and erg3B were knocked out alone and in combination. This work was funded in part by grants MPY1120/03 from the Instituto de Salud Carlos III and SAF2002-02089 from the Ministry of Science and Technology. E. Mellado held a "Ramón y Cajal" contract from the Ministry of Science and Technology. L. Alcazar-Fuoli has a predoctoral fellowship from the Instituto de Salud Carlos III. Jordi F. López's work is funded by an I3P contract from the Spanish Council for Scientific Research (CSIC). Peer reviewed