In our first case, a definitive diagnosis requires absolutely the finding of positivity for CD10 and/or renal cell carcinoma antigen and negativity for thyroglobulin and TTF-1 in areas of clear cell carcinoma in the thyroid in order to differentiate the metastatic clear cell carcinoma of kidney from the primary clear cell malignancy from thyroid. The gene discussed is TTF1; the disease is clear cell adenocarcinoma.