Further nuancing of IHC differential diagnosis in mRCC involves markers almost always negative in RCC, such as pulmonary marker TTF-1, the intestinal marker homeobox protein CDX2, p63, prostate-specific antigen, and estrogen receptor, which will be useful in excluding other carcinomas that may manifest cross-positivity for PAX8. The gene discussed is ESR1; the disease is renal cell carcinoma.