Bilateral optic nerve hypoplasia is sometimes accompanied by midline anomalies of the central nervous system, such as absent corpus callosum, absent septum pellicidum, and pituitary insufficiency.[30] None of the three genes known to produce this phenotype in animal models – namely netrin (17p13-p12), Hesx1 (3p21),[31] and DCC (18q21.3)[32] – map to either chromosome 13 or 16, suggesting an uncharacterized regulatory gene for midline CNS development in the abnormal regions. This evidence concerns the gene HESX1 and hypopituitarism.