In neurodevelopmental and psychiatric disorders including autism spectrum disorder (ASD), bipolar disorder (BD), schizophrenia, and attention-deficit/hyperactivity disorder (ADHD) altered complement profiles are observed both centrally and peripherally, with disease-specific patterns of increased or decreased C1q, C3, C4, C5, MASP-1, and C5b-9 levels correlating with behavioral phenotypes, social deficits, or cognitive impairment. This evidence concerns the gene C4A and attention deficit-hyperactivity disorder.