HAL and colorectal carcinoma: The first step of this pathway involves the enzyme histidase (EC 4.3.1.3) cleaving an amino group off l-histidine to create urocanate and ammonia as by-products, and this histidase was similarly enriched in CRC (SMD = 0.46, q < 0.1), late CRC (SMD = 0.18, P = 0.02) and metastatic CRC metagenomes (SMD = 0.35, q < 0.1).