Candida albicans WO-1 switches between a white- and an opaque-colony- forming phenotype. The gene WH11 is expressed differentially in the white phase. The WH11 open reading frame was inserted downstream of the promoter of the opaque-phase-specific gene OP4 in the transforming vector pCWOP16, and resulting transformants were demonstrated to misexpress WH11 in the opaque phase. Misexpression had no effect on the ability to switch from the white to the opaque or the opaque to the white phase, and it had no effect on the genesis of the unique opaque-phase cellular phenotype, even though the Wh11 protein was distributed throughout the cytoplasm in a manner similar to that observed for the endogenous gene product in the white phase. Misexpression did, however, increase the frequency of the opaque-to-white transition 330- fold and markedly increased the virulence of cells in the opaque phase in a mouse tail injection model.