is an emerging opportunistic pathogenic yeast capable of shifting from yeast to pseudohyphae form, and it is one of the few species with the ability to reproduce sexually. In this study, we showed that a Δ mutant, inactivated for a putative pyrophosphatase, is impaired in cell separation, pseudohyphal growth and mating. The defective phenotypes were not restored after the reconstruction of a wild-type locus, reinforcing the hypothesis of the presence of an additional mutation that we suspected in our previous study. Genetic crosses and genome sequencing identified an additional mutation in , encoding a subunit of the mediator complex that functions as a general transcriptional co-activator in Eukaryotes. We confirmed that inactivation of was responsible for the defective phenotypes by rescuing the Δ mutant with a wild-type copy of and constructing a Δ knockout mutant that mimics the phenotypes of Δ in vitro. Proteomic analyses revealed the biological processes under the control of Med15 and involved in hyphal growth, cell separation and mating. This is the first description of the functions of in the regulation of hyphal growth, cell separation and mating, and the pathways involved in .Read less <