In order to identify Yarrowia lipolytica genes induced by serum, cDNA representational difference analysis was performed using a PCR-select cDNA subtraction method. One of the genes cloned from the subtraction was a gene (YlMBF1) homologous to Saccharomyces cerevisiae MBF1 encoding the coactivator multiprotein bridging factor 1. Disruption of YlMBF1 revealed that the gene was not essential for viability, and the Ylmbf1[delta] strain did not show any distinct phenotypic change on solid serum medium. In liquid medium, however, a difference was found in the ability to maintain hyphae induced by serum. This result suggests that the YlMbf1 protein may mediate transcriptional activation of certain genes involved in the hypha formation of Y. lipolytica.