MAOA and cancer: By conducting a series of analyses focusing on tryptophan related gene expression data (see Table 1 for a list of genes analyzed) in the GEO database (gene expression omnibus) maintained by NCBI [22], we found that only Monoamine Oxidase A (MAO-A, E.C. 1.4.3.4) showed consistent decreased expression, in cancers among a variety of tissues from humans, rodents, and zebrafish.