CDX2 and gallbladder neoplasm: Tissue biopsy with immunohistochemistry is often used to differentiate metastatic lesions from primary gallbladder malignancies.8,9 Specific markers, such as CK20 and CDX2, are commonly used to confirm colorectal origin.2,10 Although gallbladder tumors typically exhibit positivity for both CK7 and CK20, the presence of CK7-negative/CK20-positive cells with CDX2 positivity is indicative of a possible colorectal primary origin.10