You’ve noticed a black line on your fingernails
If you notice a dark spot or line under one of your nails, it’s possible that you have a type of skin cancer called acral lentiginous melanoma (ALM). Acral lentiginous melanoma typically appears in places that are exposed to the sun, such as under your fingernails or on the palms of your hands, and is most common in African Americans or people with Asian heritage.