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Zona glomerulosa is the site for daily production of up to 150 mg of aldosterone, the major mineralocorticoid in humans. Steroid hormones do not bind to receptors on cellular membranes but instead modulate gene transcription by uniting with their receptors within the cell and then binding directly to DNA as the receptor-hormone complex. Each of the three zones of the cortex contains different enzymes along the three major synthetic pathways, resulting in three different classes of steroid hormones produced in each zone ( Figure 22-1). With cholesterol-derivative pregnenolone as substrate, numerous enzymes of the adrenal cortex catalyze synthesis of essential hormonal agents. Adrenal cortex and the adrenal medulla not only have distinct embryologic origins but also function as independent hormone-producing units.