Accumulation of phenylalanine and its catabolic products leads to central nervous system damage. The extent of damage caused to the brain depends on the time at which the insult occurs. Deficient myelination and abnormalities in brain proteolipids and proteins occur in late gestation and during first 6 to 9 months of life. In the fully matured brain, the synthesis of neurotransmitters is affected. This might cause nerve degeneration, behavioural difficulties and delayed development.