Abstract: A structural gene encoding a polypeptide with .DELTA.8-7 sterol isomerase activity is disclosed, Recombinant DNA molecules useful for transforming yeast and mutant yeast transformed with such recombinant DNA molecules are also disclosed. The structural gene is useful for modulating the accumulation of sterols in yeast comprising increasing the expression level of a structural gene encoding a polypeptide having .DELTA.8-7 sterol isomerase activity in a mutant yeast having single or double defects in the expression of sterol biosynthetic enzymes is provided.