Costello syndrome is a disease that produces various complications and is also caused by HRAS mutations.9, 10 Costello syndrome patients show growth and developmental delays, enlargement‐type cardiomyopathy, and frame abnormalities with dermatosis of woolly hair of the whole head, palmoplantar hyperkeratosis, papilloma, and acanthosis nigricans.11 Ninety percent of Costello syndrome patients showed HRAS p.G12S mutations. The gene discussed is HRAS; the disease is acanthosis nigricans.