Expression of aromatase in extragonadal tissues such as adipose isregulated by tissue-specific promoters, so aromatase action can generate highlocal levels of estrogen with significant biological influence, withoutsignificantly affecting circulating levels.98 The link between obesity and changes in estrogen synthesis and metabolismhave been extensively studied in the context of breast cancer. The gene discussed is CYP19A1; the disease is obesity due to melanocortin 4 receptor deficiency.