We have read with interest the recent paper by Jubb et al on the expression of hypoxia inducible factor 1α (HIF-1α) in human tumours.¹ We note that they report only 5% of ductal adenocarcinomas of the breast to be HIF-1α positive. This proportion is unusually low compared with our own data and those of other workers. In our various studies, the proportion of HIF-1α positive breast cancers varied from 44% to 80%.^2–4 In studies from other groups, these percentages varied from 56% to 76%.^5,6 We believe that this discrepancy may be caused by the use of tissue microarrays. In breast cancer, HIF-1α often shows pronounced intratumour heterogeneity because of focal perinecrotic staining, which is clinically highly relevant. Even patients with only 5% of cells overexpressing HIF-1α have a much worse prognosis.³ Thus, tissue arrays probably underestimate the true frequency of HIF-1α overexpression in breast cancer. Hence, data from studies on HIF-1α derived from tissue arrays are probably less reliable with regard to associations between HIF-1α …