For example, allopurinol, which is the most commonly used xanthine oxidase inhibitor for gout (Pacher et al., 2006), causes nephrolithiasis, hypersensitivity reaction, Stevens-Johnson syndrome, renal toxicity, allergic reactions, and fatal liver necrosis, and increases the toxicity of 6-mercaptopurin (Kong et al., 2004; Wang et al., 2004). Here, XDH is linked to gout.