Thyroid transcription factor-1 (TTF-1), also known as TITF1 or Nkx2–1, encodes a 38-kDa homeodomain-containing nuclear protein and was initially identified as an activator of thyroid-specific gene transcription.3 It is expressed in the thyroid, lungs, and brain.4 In the normal adult lung, the expression is restricted to type II alveolar cells and club (Clara) cells, which are found at the terminal respiratory unit (TRU).5–7 The expression is maintained in 62% to 76% of peripheral type adenocarcinomas.5,8–10 Therefore, TTF-1 is considered a marker of the TRU-type adenocarcinoma of the lung. The gene discussed is NKX2-1; the disease is adenocarcinoma.