One patient with epidermal nevi and HRAS p.G12S mosaic mutation was reported to develop urothelial cancer at 19 years old.15 Therefore, our patient began annual checks for malignancies at 7 years old by abdominal/pelvic ultrasonography and by urinalysis for hematuria, which is a proposed tumor screening protocol for Costello syndrome.13 Here, HRAS is linked to Costello syndrome.