HMB-45 vs. PRAME: How Do They Actually Stack Up?

ByDiskssdadras

A question I get from residents almost every week on the fellowship floor: “Do I still need HMB-45, or has PRAME made it obsolete?” The honest answer is neither stain replaces the other — they measure different things, and knowing why changes how you read your slides.

How They Differ

HMB-45 is a cytoplasmic/membranous marker against gp100, a melanosomal glycoprotein. Its major limitation in primary cutaneous melanocytic lesions is a maturation problem: benign nevi normally show strong HMB-45 positivity in the superficial component that fades with descent into the dermis — the so-called maturation pattern. That fading is expected and reassuring. The diagnostic red flag is when a lesion loses this gradient and stains diffusely and strongly throughout its full depth.

PRAME (Preferentially Expressed Antigen in Melanoma) is a nuclear stain, which on its own makes it easier to score than HMB-45’s cytoplasmic pattern — nuclear positivity is binary and harder to confuse with background pigment or histiocytes. PRAME is preferentially expressed in melanoma and largely absent in benign nevi, giving it better specificity for malignancy than HMB-45 alone. It isn’t perfect either: some Spitz nevi, blue nevi, and dysplastic nevi can show patchy PRAME positivity, so a positive result still needs architectural and clinical context, not a standalone yes/no read.

Illustration comparing a capsular nevus and metastatic melanoma in a lymph node, showing differences in HMB-45 staining intensity and location.
Image 1 (nodal capsular nevus vs. metastatic melanoma): Capsular nevus vs. metastatic melanoma within a lymph node. The capsular nevus (left) is confined to its expected location with weak-to-negative HMB-45 staining — a classic benign ‘nodal nevus’ pitfall that can mimic metastatic disease on low power. The metastatic deposits (right) occupy the parenchyma and show strong, diffuse HMB-45 positivity. Location and staining intensity together make the distinction.
Alt text: Illustration comparing the HMB-45 maturation pattern in a benign nevus to the diffuse staining pattern in melanoma.
Image 2 (maturation vs. diffuse pattern): The maturation gradient in action. In the benign nevus (left), staining intensity decreases with depth as nevomelanocytes mature. In melanoma (right), that gradient is lost — cells remain diffusely positive from top to bottom. Loss of maturation is one of the most reliable low-power clues that a lesion isn’t a garden-variety nevus.

PRAME and HMB-45, Side by Side

The comparison below puts both stains through the same two scenarios — benign nevus and melanoma — so the difference in what each marker is actually measuring becomes visual rather than abstract.

Four-panel illustration comparing PRAME and HMB-45 immunohistochemical staining patterns in benign nevus versus melanoma.
Image 3 (four-panel PRAME/HMB-45 comparison): PRAME vs. HMB-45 side by side. In a benign nevus, PRAME is negative to weak (top left) while HMB-45 shows the expected superficial-positive, deep-negative zonation (top right). In melanoma, PRAME becomes diffusely nuclear-positive throughout (bottom left) and HMB-45 loses its zonation, staining diffusely and strongly at all levels (bottom right). This is the single clearest visual summary of why the two markers are read differently — one for nuclear expression, one for depth-dependent maturation.

The Practical Takeaway

Neither stain works well in isolation for a genuinely ambiguous lesion. The literature increasingly supports combining PRAME’s nuclear pattern with HMB-45’s depth-dependent staining gradient — benign lesions are rarely diffusely positive for both, while true melanomas much more often are. Used together, they raise diagnostic specificity well beyond either stain alone.

Bottom line: HMB-45 tells you about depth-dependent maturation; PRAME tells you about nuclear expression largely restricted to malignant melanocytes. Neither is a standalone answer key — but together, read in the context of architecture and clinical history, they meaningfully sharpen the call in a genuinely borderline lesion.

Further Reading

Written by Dr. Soheil Dadras

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