PDZD9 (PDZ domain containing 9) is a 264 amino acid protein that contains one PDZ (DHR) domain and participates in protein binding. Conserved in chimpanzee, dog, cow, mouse and rat, PDZD9 exists as two alternatively spliced isoforms and is encoded by a gene that maps to human chromosome 16p12.1. Chromosome 16 encodes over 900 genes, approximately 90 million base pairs, makes up nearly 3% of human cellular DNA and is associated with a variety of genetic disorders. Giant axonal neuropathy, a nervous system disorder characterized by increasing malfunction with growth, and the rare disorder Rubinstein-Taybi syndrome, characterized by mental retardation and predisposition to tumor growth and white blood cell neoplasias, are associated with chromosome 16. Crohn's disease, a gastrointestinal inflammatory condition, and systemic lupus erythematosis are also associated with chromosome 16.