In some tumors, such as glioblastoma, bladder cancer and hepatocellular carcinoma, methylation of the promoter region of the ASS1 gene mediates its silencing; alternatively, hypoxia-inducible factor 1α (HIF1α)-mediated repression of the ASS1 promoter has also been reported in cancers such as melanoma4,8–15. Here, ASS1 is linked to cancer.