['Searching for novel molecular markers that dependably predict or indicate responses of human cancer cells to epidermal growth factor receptor (EGFR)-targeted therapy is strongly warranted. The purpose of the current study was to evaluate hypoxia-inducible factor-1alpha (HIF-1alpha) as a novel response marker compared with previously explored markers following treatment with an EGFR-blocking monoclonal antibody (cetuximab) and a small-molecule EGFR tyrosine kinase inhibitor (gefitinib) in a group of cancer cell lines containing wild-type or tyrosine kinase domain-mutated EGFR.', 'We found that, compared with previously studied response markers, including EGFR per se and three EGFR downstream signal molecules (ERK, Akt, and STAT3), which showed variable post-treatment changes in levels of phosphorylation and no consistent link of the changes to therapeutic responses, HIF-1alpha showed a selective decrease in protein levels only in responsive cell lines. To demonstrate a critical role of HIF-1alpha downregulation by EGFR-targeted treatment, we introduced a constitutively expressed HIF-1alpha mutant (HIF-1alpha/DeltaODD) that is resistant to cetuximab-induced downregulation in a cetuximab-responsive cell line (A431); we found that the HIF-1alpha/DeltaODD-transfected cells remained sensitive to cetuximab-induced inhibition of Akt and ERK phosphorylation but were remarkably less responsive to cetuximab-induced growth inhibition compared with corresponding control cells.'] Are responses of cancer cells with wild-type or tyrosine kinase domain-mutated epidermal growth factor receptor ( EGFR ) to EGFR-targeted therapy linked to downregulation of hypoxia-inducible factor-1alpha?