Estrogen synthesis in situ is believed to be primarily catalyzed by the enzyme aromatase, which is often overexpressed in breast tumors, thus explaining the increased levels of CYP19A1 mRNA in patients with locoregional breast recurrence compared with those in patients with recurrence in regions more distal to the tumor such as liver, brain and bones [15, 38]. The gene discussed is CYP19A1; the disease is breast neoplasm.