NKX2-1 and small cell lung carcinoma: CK20 and thyroid transcription factor 1 (TTF-1) have increased sensitivity and specificity for excluding small cell lung cancer,8 with CK20 being positive in 70%–100% of MCC cases and negative in small cell lung cancer, whereas TTF-1 is always negative in MCC and positive in more than 80% of small cell lung cancer cases.3, 8, 14