A new study provides new insight into the causes of trimethylaminura (TMAU), a genetically-transmitted metabolic disorder that leads to accumulation of a chemical that smells like rotting fish. Previously attributed solely to mutations in the FMO3 gene, the study identifies additional genes that may contribute to TMAU. The findings indicate that genetic testing to identify FMO3 mutations may not be sufficient to identify all underlying causes of TMAU.