['Hypoxia-inducible factor (HIF)-1alpha levels in invasive breast carcinoma have been shown to be an adverse prognostic indicator. Cellular HIF-1alpha activity is regulated by factor-inhibiting hypoxia-inducible factor 1 (FIH-1). In hypoxia, FIH-1 hydroxylation of Asn803 within the C-terminal transactivation domain does not occur and HIF-1alpha forms a fully active transcriptional complex. The present study investigates the role of FIH-1 in invasive breast carcinoma and its correlation with hypoxia.', 'Microarrayed tissue cores from 295 invasive carcinomas were stained for FIH-1, for HIF-1alpha and for carbonic anhydrase 9. FIH-1 expression was correlated with standard clinicopathological parameters and with the expression of the surrogate hypoxic markers HIF-1alpha and carbonic anhydrase 9.', 'FIH-1 was positive in 239/295 (81%) tumours, 42/295 (14%) exclusively in the nucleus and 54/295 (18%) exclusively in the cytoplasm. Exclusive nuclear FIH-1 expression was significantly inversely associated with tumour grade (P = 0.02) and risk of recurrence (P = 0.04), whereas exclusive cytoplasmic FIH-1 was significantly positively associated with tumour grade (P = 0.004) and carbonic anhydrase 9 expression (P = 0.02). Patients with tumours that excluded FIH-1 from the nucleus had a significantly shorter survival compared with those with exclusive nuclear expression (P = 0.02). Cytoplasmic FIH-1 expression was also an independent poor prognostic factor for disease-free survival.'] Is cytoplasmic location of factor-inhibiting hypoxia-inducible factor associated with an enhanced hypoxic response and a shorter survival in invasive breast cancer?