TTF1 and hereditary clear cell renal cell carcinoma: 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.