Sir,Aggressive behavior is usually an unwanted serious behavior and there are many reports in psychological medicine on the underlying genetic factors. Of several genetic polymorphisms, monoamine oxidase A (MAOA) polymorphism is widely mentioned for the clinical relationship to aggressive behavior.[1] In a recent reported, it was proved that having short polymorphism is related to the increased risk behavior and harmful practice comparing to having long polymorphism.[2] Indeed, based on the basic concern on quantum molecular genetics, the genetic molecular mass change due to different polymorphism can be expected. It is no doubt that the patients with short polymorphism have a less molecular weight of MAOA comparing to those with long polymorphism and this means less catalysis of monoamine can be expected in ones with short polymorphism. Indeed, the high accumulated level of the monoamine is proved to relate to the agitation state in psychiatricpatients,[3] and the observation might be well relating to the already mentioned effect of short versus long MAOA polymorphism. The effects of molecular mass change can be well demonstrated for case aggressive behavior and MAOA short and long polymorphism, similar to that seen in other polymorphism related medical disorders.[45]