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Mucinous adenocarcinomas (MAs) of various origins may have a similar histologic appearance and frequently metastasize to distant sites, which often causes diagnostic problems in surgical pathology practice. The immunohistochemical profiles of MAs of various origins have not been well studied. We investigated the expression of 10 immunohistochemical markers (CK7, CK20, CDX-2, β-catenin, MUC-1, MUC-2, MUC-6, ER, WT-1, and PAX-8) in 175 cases of MA, including 69 cases from the lower gastrointestinal (GI) tract, 41 from the upper GI tract, 27 from gynecologic organs, 4 from the urinary bladder, 18 from the breast, and 16 from the lung. We found that lower GI MAs (colon, rectum, and anus) frequently expressed CDX-2 (42 of 42, 100%; 33 of 42 with homogenous positivity, 79%), MUC-2 (42 of 42; 100%), CK20 (41 of 42; 98%), and β-catenin (nuclear) (27 of 42; 64%) and rarely expressed MUC-6 (2 of 42; 5%) and CK7 (8 of 42; 19%). Most of the CK7-positive cases were from the rectum and anus (7 of 8; 88%). The expression of these markers in appendiceal MAs was similar to that of low GI tract MAs, except for a lower percentage of homogenous CDX-2 (3 of 27; 11%) and nuclear β-catenin (3 of 27; 11%) expression. Unlike their lower GI tract counterparts, the upper GI tract MAs (ampulla, pancreas/biliary tree, and stomach/esophagus) frequently expressed CK7 (38 of 41; 93%) and MUC-6 (31 of 41; 76%) and were rarely homogenously positive for CDX-2 (4 of 41; 10%) and nuclear positive for β-catenin (8 of 41; 19%). Breast MAs were frequently positive for CK7 (18 of 18; 100%), MUC-1 (18 of 18; 100%), MUC-2 (18 of 18; 100%), ER (16 of 18; 89%), MUC-6 (9 of 18; 50%), and WT-1 (9 of 18; 50%). Lung MAs were frequently positive for CK7 (16 of 16; 100%) and MUC-1 (15 of 16; 94%). Gynecologic MAs were positive for CK7 (25 of 27; 93%) and PAX-8 (13 of 27; 48%). We conclude that homogenous CDX-2 and nuclear β-catenin expressions are commonly seen in lower GI tract MAs. In contrast, appendiceal MAs are usually heterogenously positive for CDX-2 and show cytoplasmic positivity for β-catenin. Unlike lower GI tract MAs, upper GI tract MAs are frequently positive for CK7 and MUC-6. As is the case in appendiceal MAs, the upper GI tract MAs may also be heterogenously positive for CDX-2. Breast MAs are positive for ER and WT-1, whereas gynecologic MAs are positive for PAX-8 and negative for WT-1.