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IHC Markers Explained

CK7 and CK20: — What the Pattern on Your Report Means

CK7 and CK20 are proteins that different tissues produce in reliably different amounts. Testing a biopsy for their combination helps a pathologist point toward where a tumour most likely originated.

Medically reviewed by Dr. Mohammed Imaduddin, Surgical Oncologist, MBBS · MS (General Surgery) · M.Ch (Surgical Oncology) · Last reviewed September 2026

  • A pattern, not a diagnosis — The four possible combinations map onto different families of cancer origin, but no single pattern confirms a site.
  • Most useful in unknown primaries — These markers are among the first ordered when cancer is found at a spread site and the starting point is unclear.
  • Always read alongside other tests — Your oncologist interprets the full IHC panel together with scan findings and clinical history — never CK7 and CK20 alone.
  • Interpretation belongs to your team — The same pattern can appear in more than one cancer type. Your oncologist weighs all the evidence before drawing a conclusion.
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CK7 and CK20 are structural proteins that normal tissues express in characteristic combinations. Pathologists test for them to help identify where a tumour most likely started. CK7-positive with CK20-negative points toward lung, breast, or ovarian origin. CK7-negative with CK20-positive points toward colorectal cancer. The pattern narrows the search — it does not confirm a diagnosis on its own.

What does each CK7 and CK20 combination suggest?

PatternCK7CK20Cancers most commonly associated
CK7+ / CK20−PositiveNegativeLung (non-squamous), breast, ovary, endometrium, bile duct, thyroid, cervix
CK7− / CK20+NegativePositiveColorectal cancer, Merkel cell carcinoma
CK7+ / CK20+PositivePositiveBladder (urothelial), pancreas, some gastric and mucinous ovarian cancers
CK7− / CK20−NegativeNegativeProstate, liver (hepatocellular), renal cell, many squamous cell carcinomas

What CK7 and CK20 can — and cannot — tell you

  • They narrow the list of possible primary sites. They do not confirm one.
  • More than one cancer type shares each pattern — the combination is a starting point, not a conclusion.
  • A positive result does not mean the cancer came from every tissue that normally expresses that protein.
  • Treatment decisions are never based on CK7 and CK20 alone.
  • Your oncologist — not the report alone — interprets what the pattern means for your specific case.

Why are CK7 and CK20 ordered?

When a biopsy shows cancer cells but it is not clear where they originated, these markers are among the first a pathologist tests. Different tissues express CK7 and CK20 in reliably different combinations, so the pattern acts as a first grid reference before more specific markers are added.

This matters most when cancer is found in a lymph node, the liver, or another spread site — places that tell you a tumour has moved but not where it started. Treatment is generally guided by where the cancer began, not only where it has spread. ASCO and ESMO guidance on carcinoma of unknown primary includes CK7 and CK20 as standard early markers in that workup.

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Terms on your report, explained plainly

Cytokeratin
A structural protein found inside the cells that line organs and glands. Different tissues produce different types, which makes them useful as markers of tissue origin.
CK7
A cytokeratin expressed mainly by glandular tissues: lungs, breast, ovary, endometrium, bile ducts, thyroid, and urinary tract. Colorectal tissue does not usually express it.
CK20
A cytokeratin expressed mainly by colorectal cells, the stomach lining, and specialised skin cells called Merkel cells. Most other glandular tissues do not express it.
IHC (immunohistochemistry)
A laboratory technique that uses antibodies to detect specific proteins in thin sections cut from a biopsy. Each result is read as positive (protein present) or negative (protein absent).
Carcinoma of unknown primary (CUP)
A cancer that has spread to one or more sites where the original starting point cannot be found on scans or initial tests. CK7 and CK20 are among the first markers used to investigate it.

Did you know?

Cytokeratins were among the first tissue-specific proteins identified in diagnostic pathology. The four-combination grid formed by CK7 and CK20 remains one of the most widely referenced starting frameworks in carcinoma of unknown primary workups.

ASCO and ESMO guidance includes it as a standard first step — not because it answers the question, but because it narrows the field before more targeted markers are ordered.

Source: ASCO and ESMO Guidelines on Carcinoma of Unknown Primary

Questions families ask about these markers

My report says CK7+ and CK20− — does that mean I have lung cancer?

Not necessarily. CK7+/CK20− is associated with lung cancer, but it is equally characteristic of breast, ovarian, endometrial, bile duct, and thyroid cancers, among others. A pathologist uses this pattern as a starting grid and then orders additional markers targeted at the most likely possibilities. No single combination confirms a site of origin. Your oncologist will explain which further tests are being done and what each is designed to distinguish.

Both markers are negative — is that a bad sign?

CK7−/CK20− is characteristic of certain cancer types, including prostate, liver, renal cell, and many squamous cell carcinomas. It is a pointer toward a particular group of tissues, not a sign that the cancer is more aggressive or advanced. Your team will order additional markers specific to those possibilities. The clinical meaning of the result depends on your full picture — scans, clinical history, and the rest of the IHC panel.

Why does the site of origin matter if the cancer has already spread?

Treatment for most cancers is guided by the tissue where the tumour began, not only by where it is found. A colorectal cancer that has spread to the liver responds differently than a cancer that started in the liver, even though both appear in the same organ on a scan. Identifying the primary site as accurately as possible allows your oncologist to apply the evidence base that most closely matches your cancer.

Can CK7 and CK20 ever give a misleading result?

Yes. Some tumours express proteins in patterns that do not match the expected grid. A poorly differentiated tumour may lose the markers of its original tissue. Some cancers express both proteins, or neither, when the standard pattern would predict otherwise. This is one reason pathologists never rely on CK7 and CK20 alone — additional markers are always ordered to cross-check the result and resolve ambiguities before a conclusion is drawn.

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Common questions

Frequently asked questions

What are cytokeratins?

Cytokeratins are structural proteins found inside the cells that line organs and glands. Different tissues produce them in different combinations, which is what makes them useful as markers in pathology. Testing a biopsy for specific cytokeratins helps a pathologist identify what type of tissue the tumour cells came from — particularly when cancer is found at a site where it may have spread rather than started.

What does CK7 positive mean on a biopsy report?

CK7 positive means the protein was detected in the tumour tissue. CK7 is expressed by several glandular tissues — lungs, breast, ovary, endometrium, bile ducts, thyroid, and urinary tract — and is typically absent in colorectal tissue. A positive result places the tumour in a broad group of possible origins. Your pathologist and oncologist use it alongside other markers to narrow that group before drawing a conclusion.

What does CK20 positive mean?

CK20 positive means the protein was detected in the tissue. CK20 is expressed mainly by colorectal cells, the stomach lining, and Merkel cells in the skin. When CK20 is positive and CK7 is negative, the pattern points toward a colorectal or Merkel cell origin. When both are positive together, the pattern points toward a different group — including bladder and pancreas. Your oncologist interprets the combination, not each marker individually.

Can the CK7 and CK20 pattern alone confirm where the cancer started?

No. The pattern narrows the possibilities but does not confirm a site on its own. More than one cancer type can share the same combination. Your oncologist interprets the pattern alongside additional markers chosen for your case, together with scan findings and your clinical history. The full picture — not any single marker — guides the conclusion. If you want to understand what the pattern showed, ask your oncologist to walk you through the report.

Do I need a new biopsy to test for CK7 and CK20?

Usually not. IHC testing is done on the tissue already taken from your original biopsy, which is preserved in wax blocks in the laboratory. The pathologist cuts thin additional sections and applies the antibody tests to them. A repeat biopsy is only needed if the original sample was too small, the tissue quality was insufficient, or a different site needs to be sampled for a specific reason your team will explain.

Who interprets CK7 and CK20 results?

A pathologist — a specialist doctor trained in laboratory diagnosis — interprets the IHC results and writes the biopsy report. Your oncologist then reads that report alongside your full clinical picture: symptoms, imaging, and medical history. The oncologist decides what the markers mean for your situation and explains the findings to you. If any part of the report is unclear, asking your oncologist to go through the key results with you is entirely reasonable.

Full index

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What Is a Biopsy?

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Types of Biopsy Compared

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Preparing for a Biopsy

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Recovery and Aftercare

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Biopsy by Body Part

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Understanding Your Report

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IHC and Molecular Markers

IHC and Molecular Markers on a Biopsy: What They Mean →

Grading and Scoring Systems

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How Accurate Is a Biopsy?

How Accurate Is a Biopsy? Errors and Second Opinions →

If Your Result Is Benign

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