Primary lung adenocarcinomas are generally positive for CK7, TTF1, Napsin A, EMA, CEA and negative for CK20, CDX2, and MUC2. TTF-1 can be useful in differentiation from metastatic carcinomas which are generally negative (except for metastatic thyroid carcinoma and some nephroblastomas). Negativity for CDX2 is useful in differentiation from metastatic colonic adenocarcinoma; however, mucinous adenocarcinomas of lung can be positive for this marker. The image shows adenocarcinoma of lung (non-mucinous type) with lepidic growth pattern. Cuboidal tumor cells line up along alveolar septa and project into the alveolar spaces. Scattered anthracotic pigment granules are present in the alveolar septa and alveolar spaces. The inset shows the tumor cells are positive for TTF-1. Image courtesy of: Jian-Hua Qiao, MD, FCAP, Los Angeles, CA, USA.