Congenital melanocytic naevus.

Congenital melanocytic naevi (CMN) are considered hamartomas or developmental malformations composed mostly of melanocytes. They often include terminal hair and other elements of skin. They are present at birth or soon after, and may become more prominent over time. They may be flat or elevated, uniform in colour or heterogeneous.
They are quite variable in size and appearance. Clinical types include:

• Small CMN: less than 1.5 cm diameter
• Medium sized CMN: 1.5 to 10 cm diameter
• Giant CMN in other sites: greater than 10 cm diameter
• Bathing trunk CMN affecting buttocks

Giant melanocytic naevi have a risk of transforming into melanoma in about 5% of patients. The risk is significantly lower in smaller lesions. It is higher in those that cross the spine, with high risk of leptomelanosis. The primary melanoma may arise within the central nervous system.