The composition and physical state of membrane lipids determine the dynamic nature of membranes, which in turn, could directly be linked to the activity of various membrane-associated cellular functions. To better understand the molecular basis of different membrane-related phenomena we established a novel strategy to alter unsaturation of mammalian cell membranes with an identical genetic background. We transfected L929 mouse fibroblastoid cells with DNA constructs containing the Delta9-fatty acid desaturase gene (Ole1) of S. cerevisiae under the control of desaturase promoters derived either from wild type or mutant strains of the dimorphic fungus H. capsulatum.