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Tyrosine hydroxylase immunohistochemistry in a catecholamine producing abdominal paraganglioma (left panel), a biochemically silent abdominal paraganglioma (middle panel) and in rat adrenal medulla (right panel). Absence of tyrosine hydroxylase immunoreactivity in the biochemically silent tumor explains the lack of catecholamine secretion and catecholamine metabolite production by that tumor. From the article by Timmers et al., in this issue, pages 4826–4832.