One type of brain protein that shows genetic differences between people is called the serotonin transporter. People can be divided by whether or not they have a short protein (S allele) or a long protein (L allele) which influences the amount of serotonin transporter. Serotonin is a key brain chemical in depression in many mental/psychiatric illnesses. We think that the genetic differences in the serotonin transporter, that may not make a difference before TBI, may become important after TBI due to the nature of these injuries.