What Is p40-Positive and What Does It Mean for a Diagnosis?

A p40-positive result on a pathology report means that cells in a tissue sample have tested positive for a specific protein that pathologists use to identify squamous cells, the flat, scale-like cells that line surfaces such as the skin, mouth, esophagus, and airways. When a tumor stains positive for p40, it is strong evidence that the growth has squamous-type differentiation, which directly shapes the diagnosis and often changes which treatments are recommended. The marker has become one of the most reliable tools in surgical pathology over the past decade, but the context around it matters enormously.

What p40 Actually Is

p40 is a shortened name for a particular version (isoform) of a protein called p63, which belongs to the same family as the well-known tumor suppressor p53. The full technical name is ΔNp63, and it is one of several isoforms produced by the p63 gene. The older, broader test for p63 picks up all of these isoforms at once, including some that are expressed in non-squamous cell types. p40 antibodies are designed to detect only the ΔNp63 isoform, which is far more restricted to squamous and myoepithelial cells. That selectivity is the whole reason p40 exists as a separate test: it tells pathologists more precisely what kind of cell they are looking at.

Why p40 Replaced p63 in Many Settings

For years, p63 was the go-to marker for identifying squamous cell carcinoma, especially in the lung. The problem was that p63 stained positive in a substantial number of tumors that were not squamous. In one widely cited study, p63 lit up in all squamous cell carcinomas (100% sensitivity), but it also stained positive in roughly a third of lung adenocarcinomas and over half of large cell lymphomas. That meant a p63-positive result could not reliably distinguish a squamous tumor from other cancer types.¹

p40 solved that specificity gap. In the same study, p40 matched p63’s perfect sensitivity for squamous cell carcinoma while flagging only about 3% of adenocarcinomas and none of the lymphomas, pushing its specificity to 98%. The practical consequence is significant: with p63 alone, a pathologist looking at a lung biopsy might mistakenly classify an adenocarcinoma or even a lymphoma as squamous cell carcinoma. With p40, that risk drops dramatically.¹ Additional studies comparing the two markers in different specimen types, including fluid samples from body cavities, have confirmed the same pattern: equivalent sensitivity but meaningfully higher specificity for p40.²

The Lung Cancer Connection

The place where p40 has had the biggest impact is in classifying non-small cell lung cancer. When a lung biopsy lands on a pathologist’s desk, one of the most important early questions is whether the tumor is an adenocarcinoma or a squamous cell carcinoma. These two subtypes respond to different drugs, and getting the classification wrong can mean giving a patient a therapy that will not work or, worse, one that carries unnecessary toxicity. Certain targeted therapies and immunotherapy combinations are approved for adenocarcinoma but not squamous cell carcinoma, and vice versa.

Pathologists now commonly use a two-marker panel of p40 and TTF-1 (thyroid transcription factor-1) to sort this out. TTF-1 is a marker for glandular (adenocarcinoma) differentiation, while p40 marks squamous differentiation. A tumor that is p40-positive and TTF-1-negative points strongly toward squamous cell carcinoma. A tumor that is p40-negative and TTF-1-positive points toward adenocarcinoma. In one study using this pairing, p40 showed 100% sensitivity and specificity for squamous differentiation, while TTF-1 had sensitivity of about 85% and specificity of 98% for adenocarcinoma.³ Using this combination on small biopsies and cell blocks, researchers have been able to correctly subtype around 85–90% of non-small cell lung cancers that morphology alone could not classify.⁴

An alternative cocktail approach uses napsin A (another adenocarcinoma marker) alongside p40 on the same slide, allowing pathologists to classify tumors with a single stain. In validation testing, this cocktail accurately subtyped 90% of non-small cell lung cancers compared with the final surgical diagnosis.⁵ This kind of efficiency matters because biopsies, especially needle biopsies, often provide very little tissue. Every stain a pathologist runs consumes some of that limited sample, so a panel that answers the question in fewer steps preserves tissue for additional molecular testing.

How Pathologists Interpret the Staining

A p40-positive result is not simply a binary yes or no. Pathologists evaluate both the percentage of tumor cells that stain positive and the intensity of the staining. According to guidelines from the International Association for the Study of Lung Cancer, diffuse and strong p40 staining in more than half of tumor cells supports a squamous diagnosis. Focal or weak p40 staining, especially in fewer than 10% of cells, should not be used to call a tumor squamous, because adenocarcinomas and other tumor types can occasionally show low-level p40 reactivity.⁶

This is a genuinely important nuance. When p40 staining falls in that gray zone between 10% and 50%, pathologists have to weigh the clinical picture, other stain results, and the intensity of the signal before making a call. In adenocarcinomas that show any p40 positivity at all, the staining tends to be very focal, limited to scattered individual cells, which looks quite different from the strong, diffuse positivity seen in true squamous tumors.⁷

A Pitfall With Small Cell Lung Cancer

One scenario that can trip up even experienced pathologists involves small cell lung cancer. Small cell tumors are biologically very different from squamous cell carcinomas, and they require a completely different treatment approach. Yet in one study, about a third of small cell lung cancers contained at least some p40-positive cells. Most of these had p40 in fewer than 1% of cells, and in the handful with slightly more, those same cells also stained for TTF-1 and neuroendocrine markers, which are hallmarks of small cell biology.⁸

The takeaway is that finding a few p40-positive cells sprinkled within a tumor does not automatically make it squamous. Pathologists need to look at the overall pattern, the percentage of cells staining, the intensity, and the results of other markers before reaching a conclusion. The guidelines around cutoff percentages described above exist precisely to avoid this kind of misclassification.

Uses Beyond Lung Cancer

While lung cancer subtyping gets the most attention, p40 has proven useful across a surprisingly wide range of organs and tumor types.

Head and Neck

Sarcomatoid carcinomas of the head and neck are aggressive tumors that can be difficult to distinguish from sarcomas (cancers of connective tissue) or even from non-cancerous reactive tissue. Both p63 and p40 detect sarcomatoid carcinomas at similar rates, but p40 is far less likely to give a false-positive result in soft tissue tumors and reactive stromal tissue. In one study, p40 stained only 5% of soft tissue tumors and none of the reactive stromal samples, compared with 30% for both categories with p63.⁹

Skin

In dermatopathology, one common diagnostic challenge is telling apart spindle cell squamous cell carcinoma from atypical fibroxanthoma, a much less aggressive spindle cell tumor that occurs on sun-damaged skin. Both can look similar under the microscope. p40 helps because it has comparable sensitivity to p63 for detecting the squamous carcinoma while being more specific, meaning it is less likely to stain the fibroxanthoma.¹⁰ When p40 is negative and the tumor has spindle cell morphology on the skin, pathologists can feel more confident leaning toward the less aggressive diagnosis.¹¹

Prostate

Prostate cancer diagnosis relies partly on demonstrating the absence of basal cells in suspicious glands. Normal prostate glands have a basal cell layer that stains positive for p63 (and p40), while malignant glands typically lack this layer. Both p40 and the older basal cell markers perform similarly in normal tissue, with staining patterns that are identical in about 88% of cases. The advantage of p40 is that it shows aberrant staining in tumor cells less often than p63, which makes it slightly more trustworthy when a pathologist is trying to confirm that a suspicious gland truly lacks basal cells.¹² In confirmed prostate adenocarcinomas, neither p40 nor the traditional high-molecular-weight cytokeratin marker stains the malignant glands.¹³

Salivary Gland Tumors

Salivary gland tumors are notoriously diverse, and several types contain myoepithelial cells that stain for both p63 and p40. But here the pattern of agreement or disagreement between the two markers becomes the diagnostic clue rather than the result of either one alone. Adenoid cystic carcinomas and pleomorphic adenomas tend to be positive for both p63 and p40. Polymorphous adenocarcinoma, a tumor that can mimic adenoid cystic carcinoma microscopically, consistently shows a p63-positive but p40-negative pattern. In one study, all 11 polymorphous adenocarcinomas tested were p63-positive and p40-negative, while 89% of adenoid cystic carcinomas were positive for both markers.¹⁴ A systematic review confirmed this concordance pattern across multiple studies.¹⁵

Cervix

Cervical cancers can be squamous or glandular (adenocarcinoma), and the distinction matters for staging and treatment. p40 has been studied as part of panels to separate these subtypes, with sensitivity around 85% and specificity above 97% for confirming squamous differentiation in cervical tumors.¹⁶

Breast

In breast pathology, both p63 and p40 serve as myoepithelial cell markers. The presence or absence of myoepithelial cells helps pathologists distinguish invasive carcinoma (which lacks them) from in situ disease and benign conditions (which retain them). Studies have found that p40 performs comparably to p63 in this role, serving as a specific and sensitive marker for the myoepithelial cell layer.¹⁷

Bladder and Urothelial Tumors

p40 is not limited to squamous cell carcinomas. It also marks urothelial carcinomas (cancers of the bladder lining) with high sensitivity, reported at 96% in one study. This is useful when a pathologist encounters a metastatic tumor of unknown origin and needs to determine whether it came from the bladder or from a squamous site.¹⁸

Why the Antibody Clone and Lab Protocol Matter

Not all p40 antibody preparations are identical. Pathology labs can choose between monoclonal and polyclonal versions of the p40 antibody, and there are multiple commercial clones available. Reassuringly, head-to-head comparisons have shown near-perfect agreement between monoclonal and polyclonal p40 antibodies, with agreement scores above 0.97. This consistency does not always hold for other markers in the same diagnostic panel; TTF-1, for example, shows more variability between different antibody clones.¹⁹ For patients and clinicians, this means that p40 results tend to be reproducible regardless of which specific lab runs the test, which is not something you can take for granted with every immunohistochemical stain.

What a p40 Result Means for Treatment Decisions

If you are reading your own pathology report and see “p40-positive,” the immediate practical question is how this shapes your treatment. In lung cancer, a squamous diagnosis confirmed by strong p40 staining generally means that certain targeted therapies commonly used in adenocarcinoma, such as drugs aimed at EGFR mutations or ALK rearrangements, are unlikely to be offered because these molecular alterations are rare in squamous tumors. Instead, treatment may lean toward chemotherapy regimens and immunotherapy combinations that have been specifically tested in squamous non-small cell lung cancer.

There is a rare and unusual scenario in which a lung tumor expresses both p40 and TTF-1 strongly, suggesting mixed squamous and glandular features. Case reports of these tumors describe high rates of TP53 gene mutations and occasional findings of other actionable mutations. These cases are uncommon and often need additional molecular testing to guide therapy.²⁰

In non-lung settings, p40 results feed into the broader diagnostic picture rather than directly dictating a drug choice. In prostate biopsies, for instance, the absence of p40-staining basal cells helps confirm malignancy, which then triggers staging workups and treatment discussions. In salivary gland tumors, the concordance or discordance between p63 and p40 can shift the diagnosis from one tumor type to another, which in turn changes the surgical approach and the expected behavior of the tumor.

When p40 Can Be Misleading

No single stain is infallible. A few situations regularly produce results that could mislead an unwary observer:

Focal staining in adenocarcinomas: A small fraction of lung adenocarcinomas show weak p40 positivity in a handful of cells. This does not mean the tumor is squamous. The staining is usually limited to isolated cells and looks obviously different from the strong, widespread staining of a true squamous tumor.
Necrotic tissue: Squamous cell carcinomas undergoing necrosis can lose p40 expression, leading to a false-negative result. The condition of the tissue and the type of fixative used can both affect staining quality.
Rare tumors with dual expression: Some tumors co-express markers that are normally mutually exclusive, such as p40 and TTF-1 together, or p40 and neuroendocrine markers. These are unusual cases that require the pathologist to step back and interpret the full panel rather than relying on any single stain.

Pathologists are trained to recognize these patterns and integrate p40 results with morphology, clinical history, and other immunohistochemical stains. A diagnosis is almost never made on a single marker alone.

p40 and Myoepithelial Cells Outside of Cancer

p40 does not only appear in the context of cancer. In normal tissue, p40 marks basal and myoepithelial cells in several organs, including the prostate, breast, and salivary glands. These cell layers act as a structural boundary around normal glands, and their presence or absence is a fundamental part of how pathologists evaluate biopsies from these sites. In benign breast disease, for example, demonstrating an intact myoepithelial layer with p40 or p63 helps rule out invasive carcinoma. In the salivary glands, myoepithelial cells staining for p40 help identify specific tumor types like pleomorphic adenoma and adenoid cystic carcinoma, where myoepithelial participation is a defining feature.²¹

The dual role of p40, as both a squamous cell marker in cancers and a myoepithelial cell marker in normal and benign tissue, is part of what makes it so versatile. But it also means the interpretation depends heavily on the clinical question being asked. A p40-positive result in a lung biopsy and a p40-positive result in a breast biopsy are answering fundamentally different diagnostic questions, even though the stain itself is identical.