Glaucomas are a group of common neurodegenerative diseases of the optic nerve and retinal ganglion cells, characterized by progressive cupping of the optic nerve head with resultant visual field loss. Elevated intraocular pressure (IOP) is a strong risk factor for glaucoma; however, glaucoma can occur at any IOP. The most common form of glaucoma in the US is primary open-angle glaucoma (POAG; see 137760). POAG that occurs with an IOP below an arbitrary threshold of 21 mm Hg is often termed 'normal tension glaucoma' (summary by Fingert et al., 2011). For a discussion of genetic heterogeneity of primary open angle glaucoma, see 137760.