E7 message is expressed in the thyroid and medullary thymic epithelial cells of the E6/E7 transgenic mouse. (a) Tissue-specific expression of the E7 transgene is detected in the thyroid only but not in the liver, spleen or whole thymus. Organs were isolated and RT-PCR was performed as described in Materials and Methods. Lanes: 1, liver; 2, spleen; 3, thyroid; 4, whole thymus. (b) Medullary thymic epithelial cells (mTECs) express E7. mTECs were isolated by cell sorting into an RNA protection solution as detailed in Materials and Methods. RT-PCR results are shown for equivalent amounts of cDNA loaded for 40 cycles. Lanes: 5, cathepsin S; 6, E7; 7, actin; 8, negative control (water).