TTF1 and lung carcinoma: Lung carcinoma typically has a CK7+/CK20− immunophenotype, while the opposite is expected of intestinal carcinoma, which generally stain CK7−/CK20+.16 In a pathology survey of 435 epithelial neoplasms, 100% of lung and 5% of colon adenocarcinomas were CK7+, while 10% of lung and 100% of colon adenocarcinomas were CK20+.17 However, primary rectum and small intestine tumors may lose CK20 expression and gain CK7 positivity.18,19 Hence, these markers must be used in combination with other markers specific for lung and intestinal tumors such as the transcription factors TTF-1 and CDX2.15