Moles form when melanocytes, the pigment-producing cells in your skin, cluster together into small groups called nests instead of spreading out evenly. Normally, melanocytes are distributed individually throughout the top layer of your skin. When something triggers them to multiply and group together, the concentrated pigment creates the visible brown or black spot you recognize as a mole. Most adults have between 10 and 40 moles, and the process behind each one involves a mix of genetics, sun exposure, and hormones.
What Happens Inside the Skin
Your skin has two main layers: the outer epidermis and the thicker dermis beneath it. Between them sits a boundary called the dermal-epidermal junction, and this is where most moles begin. Melanocytes at this junction start dividing and clustering into nests rather than staying evenly spaced. As these nests accumulate pigment, they become visible on the skin’s surface as a flat or slightly raised spot.
Moles don’t all sit in the same place within the skin, and where the nests end up determines the mole’s appearance. A mole with nests only at that junction between the epidermis and dermis tends to be flat and darker. When nests exist both at the junction and deeper in the dermis, the mole is usually slightly raised. And when nests have migrated entirely into the dermis, the mole is often dome-shaped, flesh-colored, or lighter. This progression from flat to raised reflects the natural life cycle of many moles over years or decades.
The Genetic Trigger
The clustering that creates a mole isn’t random. In most cases, it starts with a single mutation in one melanocyte that causes it to multiply. The two genes most commonly involved are BRAF and NRAS, both part of a signaling pathway that tells cells when to grow. Mutations in these genes act like a stuck accelerator, pushing the melanocyte to divide more than it should.
The most studied of these is the BRAF V600E mutation, which is found in a large proportion of common moles. Research in zebrafish demonstrated that introducing this specific mutation into pigment cells was enough, on its own, to cause the formation of mole-like clusters of melanocytes. Normal BRAF didn’t produce this effect. Only the mutated version triggered the dramatic patches of extra pigment cells that are the biological equivalent of a human mole.
The important thing to understand is that a single BRAF or NRAS mutation causes a mole to form but is not enough to cause cancer. Moles have what scientists describe as “simple genomes,” typically carrying just one driver mutation. Melanoma, by contrast, requires additional genetic hits, such as the loss of tumor suppressor genes like p53. This is why the vast majority of moles remain completely benign for life, even though they carry a mutation that sounds alarming in isolation.
Congenital Versus Acquired Moles
Moles that are present at birth are called congenital moles. They form during embryonic development, when pigment cell precursors travel from a structure called the neural crest to the skin. These precursor cells migrate along two routes: one path runs just beneath the surface of the developing skin, and another follows along nerves deeper in the body. If something goes wrong during this migration, a cluster of melanocytes can settle in one spot and form a mole before the baby is born. Congenital moles tend to carry NRAS mutations more often than BRAF mutations, and they can range from small spots to large patches covering significant areas of skin.
Acquired moles, the kind that appear throughout childhood and into adulthood, develop after birth. These are more commonly driven by BRAF mutations and tend to be smaller and more uniform in appearance. Most people develop the majority of their acquired moles between childhood and age 40, after which new mole formation slows significantly and existing moles may gradually fade or flatten.
How Sun Exposure Creates New Moles
Ultraviolet radiation is one of the strongest environmental triggers for new mole formation, particularly in childhood and adolescence. When UV light hits your skin, it damages DNA in two direct ways: it fuses together adjacent building blocks in the DNA strand, creating structural errors called photoproducts. UV also generates reactive oxygen species, unstable molecules that cause additional oxidative damage to DNA. Either type of damage can introduce the BRAF or NRAS mutations that kick off melanocyte clustering.
UV exposure also changes the chemical environment around melanocytes. When your skin absorbs UV light, both melanocytes and the surrounding skin cells release a signaling molecule called alpha-MSH, which binds to receptors on melanocytes and stimulates them to produce more pigment. This is the same process behind tanning, but it also activates growth-related pathways that can encourage melanocyte proliferation. Studies consistently show that children who get more sun exposure, especially intermittent intense exposure like sunburns, develop more moles than those with less exposure.
That said, sun exposure doesn’t explain everything. Moles frequently appear on skin that rarely sees sunlight, including the scalp, soles of the feet, and areas normally covered by clothing. This points to internal factors playing a significant role in mole formation independent of UV damage.
The Role of Hormones
Hormonal changes are another well-recognized trigger. It’s common for new moles to appear during puberty and pregnancy, two periods when hormone levels shift dramatically. Existing moles may also darken or grow slightly during these times. The exact mechanism isn’t fully mapped out, but melanocytes have receptors for several hormones, including estrogen, which can stimulate pigment production and potentially cell growth. This hormonal sensitivity helps explain why mole counts tend to increase through adolescence and why pregnant women sometimes notice changes in moles that revert after delivery.
Why Moles Eventually Fade
Despite carrying a growth-promoting mutation, moles don’t keep growing forever. Melanocytes in a mole eventually enter a state of permanent growth arrest called senescence. The same mutation that caused the initial burst of division triggers internal safety mechanisms that shut down further proliferation. This is why moles reach a certain size and stop.
Over decades, the melanocyte nests in a mole gradually migrate deeper into the dermis and lose their pigment. A mole that was dark and flat at age 15 may become raised and flesh-colored by age 50, and some moles disappear entirely in older age. This slow regression is a normal part of the mole life cycle and not a cause for concern. What does warrant attention is a mole that changes rapidly, becomes asymmetric, develops irregular borders, or shows multiple colors, as these features can signal that additional mutations have accumulated beyond the single driver mutation that originally formed the mole.
Why Some People Have More Moles
Mole count varies widely between individuals, and genetics is the biggest factor. Studies of twins show that the number of moles a person develops is strongly heritable. Fair-skinned people tend to develop more moles than those with darker skin, partly because melanocytes in lighter skin are more vulnerable to UV-induced DNA damage and partly because of inherited variations in genes related to pigment production and DNA repair. Having a variant of the MC1R gene, which is associated with red hair and fair skin, is linked to higher mole counts.
Geography and behavior matter too. People who grow up in sunnier climates or who spend more time outdoors during childhood develop more moles on average. But even among people with similar sun exposure, mole counts can differ by a factor of five or more, underscoring how much your genetic background shapes the outcome. The interplay is straightforward: your genes determine how susceptible your melanocytes are to clustering, and your environment determines how many triggers those melanocytes encounter.