In acanthosis nigricans, velvety brown hyperkeratotic plaques develop in the flexures of the axillae, groin, and neck; it may also involve the periareolar and periumbilical area.
It may occur in healthy individuals, but its appearance in the absence of usual associations (e.g., obesity, diabetes) has a strong association with genitourinary, gastrointestinal, and other internal malignancies. No treatment is required; in the palliative care setting, patients can be reassured.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. Fourth edition. Oxford University Press, 2004. (p. 350)