To the Editor: Mosaicism of an oncogenic AKT1 mutation causes Proteus syndrome, which is associated with epidermal nevi and an increased risk of cancer.(1) The occurrence of oncogenic mutations in mosaicism may increase a person's risk for malignant conditions. Somatic RAS mutations occur in 30% of tumors; germline mutations have been identified in rare developmental disorders, such as HRAS mutations in the Costello syndrome.(2) Here we report on a patient with multiple urothelial tumors reflecting mosaicism of an oncogenic HRAS mutation. A 49-year-old man presented with a widespread congenital epidermal nevus (Figure 1). When he was 19 years of age, . . .