Huntingtin (Htt) is the protein coded for in humans by the HTT gene, also known as the IT15 gene. Mutated HTT is the cause of Huntington's disease (HD), and has been investigated for this role and also for its involvement in long-term memory storage. The exact function of this protein is not known, but it plays an important role in nerve cells. Within cells, huntingtin may or may not be involved in signaling, transporting materials, binding proteins and other structures, and protecting against apoptosis, a form of programmed cell death. The huntingtin protein is required for normal development before birth. It is expressed in many tissues in the body, with the highest levels of expression seen in the brain.