There is an intimate association between sterol biosynthesis in yeast and 
aerobicity. Besides the requirement for molecular oxygen for the epoxidation of 
squalene, cytochrome hemoproteins are involved in demethylation and desaturation 
steps. Regulatory effects of hemes on sterol formation have been demonstrated 
using specifically defective mutants of yeast. Heme competency participates in a 
mechanism whereby wild-type cells are prevented from taking exogenous sterols 
from the growth media. The multiple interactions of hemes and sterols appear to 
be associated with the variously defined functions for sterols in the yeast 
cells.