BDNF and attention deficit-hyperactivity disorder: For example, abnormal levels of BDNF during early periods of brain development have been implicated in several neurodevelopmental disorders, including autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), schizophrenia, etc. Variations in the BDNF gene, including the Val66Met polymorphism, add to these risks by impacting synaptic function and cognitive outcomes [6].