Sebaceous nevi usually occur on the face or scalp, since that is where many sebaceous (oily) glands are found. [Harper, p. 1132] When the shape of a sebaceous nevus follows a pattern known as the lines of Blaschko, it is called a linear nevus sebaceous or linear sebaceous nevus. Between 1 and 3 in 1000 babies, boys and girls equally, are born with sebaceous nevi. [Rook 15.9]
Sebaceous nevi at birth are slightly raised, with a smooth or velvety surface, with colors ranging from pinkish to orange to tan. Nevi on the scalp do not grow hair. Sebaceous nevi do not spread, other than growing as the person grows. [Harper, p. 1132] However, they do tend to change at adolescence, becoming more raised and verrucous (warty). [Harper, p. 1143]
Later in life, sebaceous nevi sometimes develop tumors. While most such tumors are benign, there is a small chance of cancerous tumors forming within the nevus tissue. Surgically removing the entire nevus early in life to prevent possible future cancer is one option, though some doctors recommend that monitoring the nevus and removing any suspicious changes is sufficient. [Rook 1510; Harper, p. 1134] There continues to be debate as to whether nevus sebaceous should be surgically removed, and if so, when it should be done. [Moody, p. 15]
Sebaceous nevi are also sometimes removed for cosmetic reasons. Since sebaceous nevi do not spread, they do not return after they have been surgically removed.
There is no test to determine whether a person born with a sebaceous nevus will go on to develop LNSS. One textbook claims that of people born with sebaceous nevi, 1.7% are born with additional syndrome symptoms, and up to 10% will develop syndrome issues as children. [Rook 15.26] However, these numbers may not be accurate; the references supporting them are considerably outdated, and the syndrome is listed with the National Organization for Rare Disorders, which defines rare as affecting "fewer than 200,000 people in the United States."
If your child has a sebaceous nevus but no additional symptoms, it generally makes sense to move forward as if the nevus itself will be the only issue. Please be sure to consult with your physician!
In 2011, genetic sequencing research was able to identify mutations in two genes that are now thought to be the cause of both nevus sebaceous and linear nevus sebaceous syndrome. The mutations happen during pregnancy and are present in the nevi but not in surrounding tissues. [Groesser 783]
A collection of photographs of sebaceous nevi can be found at DermAtlas from Johns Hopkins University. (Click on an image to enlarge it, or on Next Result Set to see more photos.)