Cytokeratin 7 (CK7) is a cytoplasmic intermediate filament protein expressed on most ductal and glandular epithelium including lung, breast, bladder and female genital tract, but not in most gastrointestinal epithelium, prostate, hepatocyte and squamous epithelium. CK7 expression is absent in colon cancer, prostate cancer and squamous carcinomas. The restricted expression of CK7 in some epithelium makes it useful to identify the organ origin of adenocarcinomas when combined with staining of Cytokeratin 20 and other cell specific markers.