Tissue sections of the primary tumour and a lymph node metastasis of 60 patients with breast cancer were immunohistochemically stained for the hypoxia-markers carbonic anhydrase 9 (CA9), hypoxia-inducible factor-1α (Hif-1α) and DEC-1 and for CD34/Ki-67. This evidence concerns the gene CA9 and breast carcinoma.