The seven warning signs of skin cancer come from a clinical tool called the Glasgow 7-point checklist, designed to help identify melanoma. Three are considered major signs: a change in size, an irregular shape, and irregular color. Four are minor signs: a diameter of 7 millimeters or more, inflammation, oozing or crusting, and a change in sensation such as itching or pain. Any major sign is reason enough to get a spot checked, and even minor signs shouldn’t be ignored if they persist.
But skin cancer doesn’t always follow a neat checklist. Melanoma, basal cell carcinoma, and squamous cell carcinoma each look and behave differently on the skin. Knowing all the patterns gives you a much better chance of catching something early, when the survival rate for melanoma is above 97% for localized disease compared to just 16% once it has spread to distant parts of the body.
The Three Major Warning Signs
The Glasgow checklist gives the most weight to three features, each scoring 2 points on the clinical scale. These are the changes most strongly linked to melanoma.
Change in size. A mole or spot that’s growing is the single most important red flag. This includes both gradual enlargement over months and a sudden increase you notice over weeks. Most normal moles stabilize in size by the time you’re in your 30s or 40s, so any mole that starts expanding after that deserves attention.
Irregular shape. Normal moles tend to be roughly symmetrical. If you drew a line through the center, both halves would more or less match. A melanoma often has one half that looks nothing like the other, with edges that are ragged, notched, or blurred rather than smooth. The pigment may spread unevenly into the surrounding skin.
Irregular color. A healthy mole is usually one uniform shade of brown. A suspicious spot may contain multiple shades of black, brown, and tan, along with areas of white, gray, red, pink, or blue. The more colors present in a single lesion, the more concerning it is.
The Four Minor Warning Signs
The remaining four features each score 1 point. They’re considered less specific on their own but still meaningful, especially in combination with each other or with a major sign.
Diameter of 7 mm or more. Most melanomas are larger than about 6 millimeters (roughly the size of a pencil eraser) at the time of diagnosis. The Glasgow checklist sets its threshold at 7 mm. That said, melanomas can be tiny in their earliest stages, so size alone doesn’t rule anything out.
Inflammation. Redness or swelling around a mole or skin lesion that isn’t explained by an injury or infection can signal that something is wrong beneath the surface.
Oozing or crusting. A spot that bleeds, forms a crust, or won’t fully heal is a classic warning sign across all types of skin cancer, not just melanoma. Repeated cycles of scabbing and re-opening are especially concerning.
Change in sensation. A mole that starts to itch, tingle, burn, or feel tender is worth watching. Melanoma usually isn’t painful in its early stages, but itching and bleeding tend to appear as it progresses.
How to Use the ABCDE Rule Alongside the Checklist
The Glasgow 7-point checklist and the ABCDE rule overlap significantly, but the ABCDE framework, developed by the National Cancer Institute, adds one important concept: evolution. Here’s how the five letters break down:
- Asymmetry: One half doesn’t match the other.
- Border: Edges are ragged, notched, or blurred.
- Color: Multiple shades or unexpected colors like blue, white, or red.
- Diameter: Larger than 6 mm, though melanomas can start smaller.
- Evolving: Any change in size, shape, color, or behavior over the past few weeks or months.
The “Evolving” criterion is the most practical one for self-checks. If a spot is changing in any way, that matters more than whether it perfectly matches the other criteria.
The Ugly Duckling Sign
If you have many freckles or moles, there’s a simpler screening concept that works well alongside formal checklists. The “ugly duckling” sign means looking for the one spot that doesn’t match the rest. Maybe it’s more raised than your other moles, has a different color, or has scabbed over. Most of your moles will look similar to each other. The one that stands out is the one to get examined.
What Other Skin Cancers Look Like
The 7-point checklist and ABCDE rule are designed for melanoma, but two other types of skin cancer are actually more common.
Basal cell carcinoma, the most frequently diagnosed skin cancer, doesn’t look much like a mole. It typically appears as a shiny, translucent bump with a pearly or waxy surface. On lighter skin, it looks white or pink. On darker skin, it often appears brown or glossy black. You may notice tiny blood vessels running through it. These growths tend to bleed easily, scab over, and then reopen. A sore on sun-exposed skin that won’t heal after several weeks is a hallmark presentation.
Squamous cell carcinoma, the second most common type, has a different appearance. It often shows up as a firm bump or nodule that may be pink, red, brown, or skin-colored. It can also present as a flat sore with a scaly crust, a rough scaly patch on the lip, or a wart-like growth. A new raised area developing on an old scar is another pattern to watch for. Squamous cell carcinoma can also appear inside the mouth as a sore or rough patch.
Precancerous Spots to Watch
Actinic keratoses are rough, scaly patches caused by years of sun exposure. They typically look like small pink spots, but the texture is often more noticeable than the appearance. They feel gritty, like sandpaper, and you may feel them before you can see them. Estimates suggest 1% to 10% of these spots eventually develop into squamous cell carcinoma, so treating them early is a straightforward way to reduce your risk.
Places Skin Cancer Hides
Most people check their arms, chest, and face but skip the areas where skin cancer can grow undetected for months or years. Your scalp is vulnerable, especially where you part your hair or where hair has thinned. The soles of your feet and the spaces between your toes are commonly overlooked, yet a specific type of melanoma called acral lentiginous melanoma develops in exactly those locations. This form is most common in people of color.
Nail beds are another hiding spot. Both melanoma and squamous cell carcinoma can form under fingernails and toenails, appearing as dark streaks or discoloration beneath the nail. If you regularly wear nail polish, it’s worth checking your bare nails periodically to make sure nothing is developing underneath.
Sensations That Signal a Problem
Not every warning sign is visual. Some skin cancers produce sensations you can feel: persistent itching, tenderness, a burning feeling, or outright pain. A spot that itches or hurts without an obvious cause, or one that bleeds spontaneously, is worth having evaluated. These sensory symptoms can appear in melanoma, basal cell carcinoma, and squamous cell carcinoma alike. The pattern to watch for is persistence. A bug bite heals. A scratch scabs over and resolves. A skin cancer keeps coming back or never fully goes away.