What Are Moles (Nevi)?

Most people have at least a few moles, or nevi – those small brown spots that seem to appear out of nowhere and stick around for life. Although moles are one of the most common things dermatologists examine, there’s still a lot of confusion about why they form and when they should be checked. So what exactly are moles, why do some people develop more than others, and how can you tell when a mole needs medical attention? Your safest option is tofind a board certified dermatologist near you.

Here, we break down the science behind what a mole is and when it might be concerning.

A mole (or nevus) forms when melanocytes, the pigment-producing cells in the skin, cluster together instead of spreading evenly throughout the epidermis. These melanocytes produce melanin, the pigment that gives your skin color, which is why moles appear brown, tan, or black.

Most moles form during childhood and adolescence, when the skin is actively developing. During this time, melanocytes are more responsive to genetic signals, hormonal changes, and UV exposure, which can all influence how many moles you develop.

Factors that contribute to mole formation include:

• Genetics: If your parents have many moles, chances are you will, too.

• Sun exposure: UV light stimulates melanocyte activity and can trigger new moles, especially during childhood.

• Skin tone: People with fair skin typically develop more moles than those with darker skin tones.

• Hormonal shifts: Puberty, pregnancy, and certain medications that affect hormones can cause moles to darken or change.

• Immune response: Conditions that affect immune regulation can affect how moles form or change, since melanocytes also respond to immune signaling within the skin.

Moles vary widely in appearance, but most have several common characteristics:

• Color: Usually uniform tan, brown, or black, depending on melanin concentration.

• Shape: Often round or oval with smooth, well-defined borders.

• Size: Usually under 6 mm in size, but congenital or atypical moles can be larger.

• Texture: Flat, slightly raised, or dome-shaped.

• Hair: Some moles grow a hair or two, which is normal and not a sign of cancer.

Normal moles tend to look the same over long periods. Sudden changes in any of these characteristics can be a red flag and something your dermatologist should take a look at.

Most moles are completely harmless. However, some types of skin cancer, especially melanoma, can resemble moles, which is why monitoring them is so important.

Melanoma occurs when melanocytes undergo genetic mutations that cause them to grow in an uncontrolled, abnormal way. These mutations can be driven by factors like UV damage, inherited genetic variants, or impaired immune function. While melanoma can develop inside an existing mole, a large percentage of melanomas actually appear as brand-new lesions on previously normal skin. This is why it is so important to watch for any new spot that looks unusual, in addition to monitoring moles you already have.

To help identify suspicious moles, look for the “ABCDEs” of melanoma:

• A – Asymmetry: One half doesn’t match the other.

• B – Border: Edges are irregular, scalloped, or blurred.

• C – Color: Multiple colors or unusual shades (black, red, white, blue).

• D – Diameter: Larger than 6 mm, though melanomas can be smaller.

• E – Evolving: Any change in size, color, shape, itching, crusting, or bleeding.

Melanoma risk increases if you have:

• A large number of moles (especially over 50).

• Atypical (dysplastic) moles.

• A family history of melanoma.

• History of severe sunburns, especially in childhood.

• Fair skin, light eyes, or freckles.

• Immunosuppression.

There are different types of nevi, classified based on behavior, appearance, and where melanocytes are clustered within the skin. Mole types include:

• Common acquired nevi. These are the everyday moles most people have. They appear in childhood or early adulthood and are generally small, uniform in color, and benign.
• Atypical or dysplastic nevi. These moles look different from typical ones and are often larger, with irregular borders and variegated color. While not cancerous themselves, they indicate higher melanoma risk and should be monitored more closely.
• Congenital moles. These moles are present at birth and can range from tiny spots to large patches. Larger congenital nevi carry a higher lifetime risk of developing melanoma
• Blue nevi. These are deep blue or slate-gray spots caused by melanocytes located deeper in the dermis. They’re typically benign but can be mistaken for melanoma because of their color.
• Junctional nevi are located at the junction of the epidermis and dermis and are usually flat and dark.
• Compound nevi are located in both the epidermis and dermis and are often slightly raised.
• Intradermal nevi lie deeper and are often flesh-colored or dome-shaped.

There are also skin bumps that people call moles such as:

• Acrochordons are benign "skin tags" occur in areas of friction.
• Seborrheic keratosis are warty looking skin growths that can be dark and look like a mole.

Most true moles would require excision in order to remove them. Dermatologists do not typically recommend removing moles unless there is a skin cancer concern or other medical concern, such as the mole getting snagged on clothing feeling uncomfortable.

When your mole is removed, it will usually be sent to pathology to look under the microscope to make sure it is benign.

Some skin-lightening skin care ingredients can help to even skin tone and lighten dark spots and may help to prevent the overproduction of melanin. 

Some popular skin-brightening products include:

Not entirely. Genetics play a major role in their development, so many are unavoidable. However, you can reduce the formation of sun-induced moles and significantly decrease your melanoma risk by practicing good sun protection habits such as wearing daily sunscreen, avoiding peak UV hours, and avoiding tanning beds.

You should schedule a skin check if you notice:

• A brand-new mole after age 30.

• Any mole that has changed in size, shape, or color.

• Itching, crusting, or bleeding.

• A mole that looks significantly different from your others (“the ugly duckling sign”).

• A large number of moles or a personal/family history of melanoma.

Your dermatologist may recommend a biopsy of suspicious-looking moles, which is a quick and routine procedure that involves taking a small sample of the mole to examine under a microscope and determine whether any abnormal or cancerous cells are present.

Moles are very common and usually harmless. They’re clusters of pigment-producing cells influenced by genetics, UV exposure, and hormones. While most moles stay benign for life, some types of melanoma can mimic a normal mole, so monitoring your skin and understanding the warning signs is essential. With smart sun habits and regular dermatology visits, you can protect your skin and catch concerns early.

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