Immunohistochemistry assays revealed a marked over-expression of Fra-1 and c-Fos in 100% of 210 tissue samples from different human breast tumors [invasive ductal carcinoma (n = 200), Medullary carcinoma (n = 2), Phyliodes sarcoma (n = 2), Mucinous carcinoma (n = 2), lobular carcinoma in situ (n = 2) and squamous cell carcinoma (n = 2)], contrasting with the low or undetectable levels of Fra-1 and c-Fos detected in the non-pathological samples (n = 37). The gene discussed is FOSL1; the disease is medullary thyroid gland carcinoma.