Serotonin N-acetyltransferase (SNAT) (EC 126.96.36.199), also known as arylalkylamine N-acetyltransferase (AANAT) is an enzyme that is involved in the day/night rhythmic production of melatonin, by modification of serotonin. It is in humans encoded by the ~2.5 kb AANAT gene containing four exons, located on chromosome 17q25. The gene is translated into a 23 kDa large enzyme. It is well conserved through evolution and the human form of the protein is 80% identical to sheep and rat AANAT. It is an acetyl-CoA-dependent enzyme of the GCN5-related family of N-acetyltransferases (GNATs). Serotonin N-acetyltransferase may contribute to multifactorial genetic diseases such as altered behavior in sleep/wake cycle and research is on-going with the aim of developing drugs that regulate AANAT function.