AMPD2 and primary optic atrophy: Finally, in eight new PCH9 patients and 17 previously reported individuals with biallelic AMPD2 variants, Kortüm and colleagues [55] detected cases that lacked the “figure-8” midbrain shape, as well as a lack of visual contact, central visual impairment, a pale optic disc, or primary optic atrophy, again highlighting the phenotypic heterogeneity in human disease.