A biological factor that causes autism may have a dual function. A factor that causes autism (shown in red) may be expressed (1) in the CNS, where it plays a role in the early development of the brain, and (2) outside the CNS, where it participates in processes that determine the 5-HT levels in blood platelets. The "central" and "peripheral" 5-HT systems are separated by the blood-brain barrier (BBB) that matures after birth. It is usually not clear until age 2 or 3 whether the brain is autistic (black box). By that time, the factor has altered numerous developmental processes in the brain and may no longer be obvious. This same factor continues to operate years after birth outside the CNS, where it maintains higher than normal 5-HT levels in blood platelets. In contrast to the brain, blood platelets are short-lived and continue to be produced throughout the life span.