Aristolochic acid is produced in Aristolochia plants, which are components of herbal medicines commonly used in Asia. Aristolochic acid was classified as a carcinogen after exposure was associated with upper urinary tract urothelial carcinoma (UTUC). Now, two groups separately determine the mutational signature of aristolochic acid. Bin Teah Teh, Patrick Tan, Steven Rozen, See-Tong Pang and colleagues report whole-genome sequencing of one aristolochic acid–exposed UTUC tumor and exome sequencing of eight additional exposed tumors (Sci. Transl. Med. 5, 197ra101, 2013). Separately, Thomas Rosenquist, Kenneth Kinzler and colleagues report exome sequencing of 19 aristolochic acid–exposed UTUCs and 7 non-exposed UTUCs (Sci. Transl. Med. 5, 197ra102, 2013). Both groups observed a very high mutation rate in exposed tumors and identified recurrent somatic mutations in genes that encode chromatin modifiers. They both determined that the aristolochic acid mutation signature is characterized by A:T to T:A transversions that preferentially occur at a specific sequence motif. This sequence motif occurs in the canonical splice acceptor site, and both groups found an over-representation of splice-site mutations. These papers shed light on the mutagenic properties of this carcinogen in herbal remedies.