The production of germ tubesresults in the conversion to a filamentous growth phase orhypha, also called the mycelial form. The formation ofpseudohyphae occurs by polarized cell division when yeast cellsgrowing by budding have elongated without detaching fromadjacent cells. Under certain nonoptimal growing conditions,C. albicans can undergo the formation of chlamydospores,which are round, refractile spores with a thick cell wall. Thesemorphological transitions often represent a response of thefungus to changing environmental conditions and may permitthe fungus to adapt to different biological niches. The transition from a commensal to a pathogenic lifestyle may also involve changes in environmental conditions and dispersionwithin the human host. The ultrastructure, composition, andbiological properties of the cell wall are affected by thesemorphological changes (64). Although progress has beenachieved in the recent years, the molecular mechanisms governing these morphogenetic conversions are still not fully understood, partly due to the difficulty of genetic manipulationsin this fungus (274, 275, 474). Recent reports that may heraldrapid advances in this area have identified transcriptional regulatory genes, a general transcriptional repressor TUP1 (38), aputative transcriptional factor RBF1 (220), and a myc-like transcriptional factor EFG1 (516) that affect cellular morphologywhen their expression is altered. Most of the observations fromthese studies have been incorporated by Magee (310) into amodel for the regulation of pseudohyphal growth.