The enzymes catalyzing specific reactions within steroidogenic cells are localized in the membranes of either the mitochondria or the smooth endoplasmic reticulum or SER. Specific enzymatic reactions occur because steroid intermediates diffuse between these two compartments as well as out of the cells. In the case of estrogen biosynthesis, the reaction sequence occurs in two different cell types as well as two sub-cellular compartments.

Cytoplasmic cholesterol is transferred from the outer mitochondrial membrane to the inner mitochondrial membrane by steroid acute regulatory protein or StAR. Transfer of cholesterol from the outer to inner mitochondrial membrane is the rate limiting step in steroid hormone biosynthesis. Mutations in the StAR gene cause the potentially lethal disease, lipid congenital adrenal hyperplasia.

In this condition, the cell fills with cholesterol and cholesterol esters because transfer across the mitochondrial membrane is blocked.