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IHC Markers Explained

S100, SOX10 and Melan-A: — What These Markers Actually Mean

An IHC report with a plus sign next to S100, SOX10 or Melan-A can look like a code. Each plus means the tumour cells express that protein. What it tells your pathologist — and what it cannot tell you on its own — is what this page explains.

Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed September 2026

  • Positive means expressed — A plus sign means the laboratory detected that protein in the tumour cells.
  • No marker works alone — Pathologists combine several stains. The pattern across the panel is what points to a diagnosis.
  • S100 is sensitive, not specific — S100 appears in many normal tissues, so a positive result opens a question rather than answering it.
  • SOX10 and Melan-A narrow the field — These are more selective, which is why they are added alongside S100.
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S100, SOX10 and Melan-A are proteins that pathologists look for to identify cells of melanocyte or nerve origin. A positive result means the tumour cells express that protein. No single marker is read alone — your pathologist combines the full panel to reach a diagnosis, and your oncologist interprets what it means for your treatment.

What does each marker mean?

S100
A calcium-binding protein present in melanocytes, nerve-sheath cells, cartilage, fat and several other normal tissues. Highly sensitive but not specific — it appears across too many tissue types to name a tumour on its own.
SOX10
A transcription factor that controls how neural crest cells develop. More selectively expressed than S100 in melanoma and nerve-sheath tumours, making it useful for separating these from carcinomas.
Melan-A (MART-1)
A protein found almost exclusively in melanocytes and the tumours that arise from them. More specific for a melanocytic origin than S100 — its presence alongside S100 and SOX10 points more strongly toward a melanocytic tumour.
HMB-45
A melanocyte marker often reported alongside Melan-A. May be negative in deeply invasive or spindle-cell variants of melanoma, which is one reason pathologists use a panel rather than a single stain.

Why are these markers reported together?

Pathologists read the whole panel, not each marker in isolation. The pattern across stains is what points toward a diagnosis.

S100, SOX10 and Melan-A all positive is consistent with melanoma or another melanocytic tumour. S100 and SOX10 positive with Melan-A negative more commonly points toward a nerve-sheath tumour — though spindle-cell melanoma can also be Melan-A negative.

A negative on one marker adjusts probability; it does not overturn the conclusion. Your pathologist's written diagnosis weighs all of this. That conclusion matters more than the individual plus or minus signs.

What to keep in mind when reading your report

  • Read the pathologist's conclusion — it is the interpreted answer, not the list of stains.
  • A positive S100 alone is not a melanoma diagnosis.
  • A negative on one marker in the panel does not mean the diagnosis is wrong.
  • Ask your oncologist which markers were decisive for your specific tumour type.
  • If a result seems to contradict what you were told verbally, raise it at your next appointment.

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How do these markers compare?

MarkerSensitivity for melanomaSpecificity for melanomaAlso positive in
S100Very highLow — broadly expressedNerve-sheath tumours, some carcinomas, cartilage and fat tumours
SOX10HighModerate — more selective than S100Schwannomas, neurofibromas, some salivary gland tumours
Melan-A (MART-1)HighHigh — melanocyte-selectiveAdrenocortical tumours, some angiomyolipomas
HMB-45ModerateHigh — but may be negative in spindle-cell melanomaAngiomyolipomas, some adrenal tumours

Did you know?

S100 takes its name from its solubility in a saturated ammonium sulphate solution — a laboratory property, not a clinical one.

Because it is expressed in more than a dozen normal tissue types, pathologists almost never rely on it alone to diagnose melanoma.

Source: College of American Pathologists

Questions this report often raises

Does a positive S100 mean I have melanoma?

Not on its own. S100 appears in many normal tissues — nerve sheaths, cartilage, fat and more — so it confirms neural crest origin, not a specific tumour type. SOX10 and Melan-A are needed to narrow the field further. Your pathologist's conclusion reflects the whole panel, not the S100 result alone.

Which tumours show all three markers positive?

S100, SOX10 and Melan-A all positive is most consistent with a melanocytic tumour — cutaneous melanoma, mucosal melanoma, or occasionally a melanocytic naevus sent for assessment. Clear-cell sarcoma can show a similar staining profile. Your pathologist considers the tumour's appearance under the microscope and your clinical history alongside the marker results before reaching a conclusion.

What if S100 and SOX10 are positive but Melan-A is negative?

This pattern more commonly points toward a nerve-sheath tumour — such as a schwannoma or malignant peripheral nerve sheath tumour — rather than a melanocytic one. However, spindle-cell variants of melanoma can also be Melan-A negative. This is one reason pathologists may add further stains or reference the tumour's histological appearance before stating a final diagnosis.

Why does my panel differ from someone with the same diagnosis?

Panels are chosen based on how the tumour looks under the microscope and the specific diagnostic question at that stage. A spindle-cell tumour on the skin calls for a different starting panel than an undifferentiated node metastasis. What matters is the conclusion the panel supports — not the particular markers used to get there. Ask your oncologist to explain the diagnostic reasoning if it is unclear.

Can these markers help decide my treatment?

Marker results contribute to the diagnosis, and the diagnosis informs treatment — but the step from a positive Melan-A to a specific treatment plan belongs to your oncologist. Staging, molecular profiling, your general health and national guidelines all feed into that decision. Marker results alone are not a basis for starting, changing or delaying treatment without speaking to your treating team.

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Common questions

Frequently asked questions

What does a positive IHC result actually mean?

A positive result means the laboratory detected the target protein in the tumour cells. The stain attaches to an antibody seeking that protein — if the protein is present, the tissue colours and the pathologist records it as positive. A negative result means the protein was not detected at the test's level of sensitivity, not necessarily that it is absent in every cell.

Is a melanoma diagnosis certain if all three markers are positive?

A triple-positive result — S100, SOX10 and Melan-A — is strongly consistent with a melanocytic tumour, but your pathologist reads this alongside tumour morphology, location and clinical history before stating a diagnosis. IHC supports the pathologist's judgement; it does not replace it. The written conclusion at the end of the report is the authoritative answer.

My report says 'focally positive' — is that different from just positive?

'Focally positive' means the stain was detected in only a small proportion of the cells, not throughout the tumour. Pathologists note the extent because it affects how much weight to give the result. Ask your oncologist what the distribution of staining meant for your specific report — it is a reasonable question to ask directly.

Why is S100 used in so many panels if it is not specific?

Sensitivity matters at the screening stage. When a pathologist encounters an unfamiliar tumour, a broadly sensitive marker like S100 helps narrow the likely tissue origin before more specific stains are added. It reliably flags neural and melanocytic lineages among many possibilities, guiding the next round of testing. Specificity comes from the panel as a whole, not from any single marker.

Can IHC markers be used to monitor my response to treatment?

IHC on tissue is a diagnostic tool, not a monitoring test — it captures protein expression at the time the biopsy was taken. Treatment response is assessed through scans, blood markers and clinical examination. If a new biopsy is taken during or after treatment, IHC may form part of re-evaluating the tumour, but that is a separate clinical decision for your oncologist.

Where can I get my IHC report explained in detail?

Your treating oncologist is the right person to walk through your full report and explain what each result means for your diagnosis and plan. At CION, pathology is reviewed alongside your scans and clinical history — the only context in which individual markers make complete sense. If the explanation you received was unclear, ask for a focused session and bring someone to help take notes.

Full index

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IHC and Molecular Markers

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