CDX2

CDX2 (Caudal-type homeobox 2) is a homeobox transcription factor of the ParaHox family, encoded by the CDX2 gene on chromosome 13q12.2. It acts as a lineage-specifying factor for the intestinal epithelium (from duodenum to colon), driving and maintaining intestinal differentiation programs. CDX2 is essential for intestinal development, establishment of intestinal cell phenotype and maintenance of adult gut epithelial identity, where it regulates both proliferation and differentiation of intestinal epithelial cells. Physiologically, CDX2 shows strong nuclear expression in normal small and large intestinal epithelia, while it is absent in most non-intestinal epithelia.

CDX2 is one of the most sensitive immunohistochemical markers for colorectal adenocarcinoma in both primary and metastatic settings. It is routinely included in panels, often together with CK7, CK20 and SATB2, to help determine the site of origin in carcinomas of unknown primary, particularly when a gastrointestinal source is suspected. CDX2 is also useful for classifying midgut neuroendocrine tumors and for demonstrating intestinal differentiation in various neoplasms. Because of its limited specificity being an intestinal lineage marker, CDX2 must always be interpreted as part of a broader immunohistochemical panel.

• Colorectal adenocarcinoma (CRC) CRC typically shows diffuse, strong nuclear CDX2 expression and present in 80-95% of CRCs. Loss of CDX2 may occur in poorly differentiated tumors, MSI-high and right-sided colon cancers. CDX2 is widely used to confirm intestinal/colorectal origin in metastases and when paired with SATB2 (or Cadherin 17), enhances specificity for the lower gastrointestinal tract. The immunoprofile CDX2+/SATB2+/Cadherin17+/CK20+/CK7− is highly specific for CRC.
• Mucinous pelvic neoplasms In mucinous pelvic neoplasms, particularly peritoneal mucinous disease or mucinous tumors involving the ovary, diffuse CDX2 expression favors an appendiceal or lower gastrointestinal origin over a primary ovarian source. However, CDX2 expression is observed in 20–50% of mucinous ovarian carcinomas, which creates a diagnostic challenge in distinguishing these from metastatic colorectal carcinoma. Reliable classification therefore requires a panel-based approach, with markers such as SATB2 providing greater specificity for colorectal origin.
• Gastric carcinoma with intestinal metaplasia Intestinal-type gastric carcinoma and areas of intestinal metaplasia are commonly CDX2-positive, whereas diffuse-type gastric carcinoma is less frequently positive. CDX2 thus aids in subtyping gastric lesions.
• Neuroendocrine tumors (NETs) NETs show variable CDX2 expression depending on their site of origin. Midgut NETs (ileum, appendix, right colon) are typically diffusely CDX2-positive, reflecting their derivation from CDX2-driven intestinal enteroendocrine cells. Hindgut NETs (rectum, left colon) are much less often CDX2-positive, usually focal or weak, consistent with the fact that L-cell–derived endocrine cells in this region do not express CDX2. Foregut NETs, including those of the lung, thymus, stomach and pancreas, are generally CDX2-negative.
• Pancreatic ductal adenocarcinoma (PDAC) A subset of PDAC, particularly intestinal-type and Intraductal Papillary Mucinous Neoplasm-related, may show weak or heterogeneous CDX2 staining reaction, in contrast to the strong, diffuse pattern typical of colorectal carcinoma. CDX2 positivity therefore does not exclude a pancreatobiliary origin and must be interpreted alongside markers such as SMAD4.
• Gynecologic endometrioid lesions with morules (pitfall) Morule-forming endometrioid proliferations may exhibit diffuse CDX2 expression that is unrelated to intestinal differentiation, whereas the glandular components are typically negative or only focally positive. This pattern should not be mistaken for colorectal origin, and in cases of diagnostic uncertainty, correlation with SATB2 is recommended.
• Prostatic Adenocarcinoma (pitfall) Although uncommon, CDX2 expression has been reported in prostatic adenocarcinoma—both in primary tumors (approximately 5% of primary acinar adenocarcinomas and up to 36% of primary mucinous types, typically showing focal and weak staining) and in about 10% of metastatic cases. This finding represents a potential diagnostic pitfall and should always be interpreted in conjunction with prostate-specific markers such as NKX3.1.

Pancreas is recommended as positive tissue control for CDX2, as it displays a low level of detection (LLOD). Virtually all ductal and intercalated duct epithelial cells should demonstrate at least weak to moderate, distinct nuclear staining reaction. In contrast, appendix and colon are not suitable as primary positive tissue controls because their epithelial cells express high levels of CDX2, which will not be applicable to monitor IHC test reproducibility and identify a result with reduced analytical sensitivity and potential false negative results with an otherwise correctly validated IHC assay. Tonsil serves as an appropriate negative tissue control: no nuclear or cytoplasmic staining reaction should be observed in endothelial or smooth muscle cells and the majority of lymphocytes must remain without CDX2 reaction, although weak nuclear reactivity may occasionally be seen in scattered lymphatic cells.