Acanthosis nigricans is a skin condition in which brown or black velvet-like markings appear under the arms, in the groin, or on the back of the neck. Any skin fold can be affected, including the lower lip and chin.

Causes of acanthosis nigricans may include:

  • High insulin levels in people who are obese
  • A family history of acanthosis nigricans
  • Rarely, a cancerous tumor