In this study, firstly using a specific anti-human CYP2C9 antibody in Western Blotting, which does not crossreact with other CYP2C-members (CYP2C8, CYP2C18, CYP2C19), we could also show weak expression of CYP2C9 protein in all human breast tumours tested. The gene discussed is CYP2C18; the disease is breast neoplasm.