Acanthosis nigricans is a brown to black, poorly defined, velvety hyperpigmentation of the skin. It is usually found in body folds, such as the posterior and lateral folds of the neck, the armpits, groin, navel, forehead, and other areas. Acanthosis nigricans is conventionally divided into benign and malignant forms, although may be divided into syndromes according to cause: Benign This may include obesity-related, hereditary, and endocrine forms of acanthosis nigricans. Malignant. This may include forms that are associated with tumour products and insulin-like activity, or tumour necrosis factor.