Arginase-1 is an enzyme that catalyzes the hydrolysis of arginine to ornithine and urea in the urea cycle. In normal tissues, arginase-1 is primarily expressed in hepatocytes. Recent investigations have reported that the vast majority of hepatocellular carcinomas express this marker, but it is found only rarely in nonhepatocellular tumors. Owing to its restricted expression in hepatocellular carcinomas, arginase-1 has proved to be a useful immunohistochemical marker for assisting in distinguishing between these tumors and other neoplasms with which they may be confused.