Phenotypic transitions play critical roles in host adaptation, virulence, and sexual reproduction in pathogenic fungi.A minority of natural isolates of Candida albicans, which are homozygous at the mating type locus (MTL, a/a or α/α), are known to be able to switch between two distinct cell types: white and opaque.It is pu77ling that white-opaque switching has never been observed in the majority of natural C.albicans strains that have heterozygous MTL genotypes (a/α), given that they contain all of the opaque-specific genes essential for switching.