A nevus is caused by cells called melanocytes, which produce the pigment that colors skin, hair and eyes. Most of the time, melanocytes are distributed evenly, but occasionally they form clusters, which leads to the development of nevi.
Most nevi are present at birth or develop within the first two decades of life, but they can continue to develop into mid-life. Caucasians are more likely to develop nevi than other races. It has been estimated that choroidal nevi occur in about 6 percent to 7 percent of Caucasians.
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