**Core Concept:** Carcinoembryonic antigen (CEA) is a glycoprotein that is normally produced by the gastrointestinal tract during fetal development and is also expressed by some gastrointestinal malignancies. CK7 and CK20 are cytokeratins, which are intermediate filament proteins expressed in specific cell types. CK7 is expressed in a variety of epithelial cells, including those of the lung, thyroid, and fallopian tube, while CK20 is expressed in colorectal, pancreatic, and urinary tract epithelial cells.

**Why the Correct Answer is Right:** In this question, we are discussing a tumor that is negative for both CK7 and CK20 immunohistochemistry (IHC) staining. This indicates that the tumor is not of respiratory, thyroid, or fallopian tube origin, as CK7 is expressed in these tissues. Additionally, the tumor is not of colorectal, pancreatic, or urinary tract origin, as CK20 is expressed in these tissues.

**Why Each Wrong Option is Incorrect:**

A. CK7-negative and CK20-positive: This tumor could be of respiratory, thyroid, or fallopian tube origin, as mentioned earlier.

B. CK7-negative and CK20-negative: This option is incorrect because the tumor is expected to express a cytokeratin, either CK7 or CK20, based on the fact that they are expressed in specific epithelial cell types.

C. CK7-positive and CK20-positive: This option is incorrect because the tumor is expected to express either CK7 or CK20, not both.

D. CK7-positive and CK20-positive: This option is incorrect because the tumor is expected to express either CK7 or CK20, not both.

**Clinical Pearl:** The combination of CK7-negative and CK20-negative staining pattern is suggestive of a gastrointestinal tumor, as these tumors are CK7-negative and CK20-negative by definition. This combination helps in narrowing down the potential origin of the tumor, leading to a more targeted diagnostic and therapeutic approach.