PAX8 and neoplasm with perivascular epithelioid cell differentiation: Although the tumors in our STP model were epithelioid and definitively derived from a PAX8-positive cell of origin due to use of the Pax8-CreERT model33, the lack of keratin and renal tubular marker expression raised the possibility that these tumors more closely resembled malignant PEComas rather than tRCC.