Together with the enrichment in ADHD of neurodevelopmental copy number variations implicated in schizophrenia (e.g., 22q11.2, NRXN1) [45], these observations support a genetic-stratification hypothesis: ADHD–psychotic and ADHD–bipolar lineages may reflect partially distinct allele constellations. This evidence concerns the gene NRXN1 and schizophrenia.