ELP4 and optic nerve disorder: Moreover, the fact that mutations and/or variants in ELP2-ELP4, are associated with distinct neurological disorders (albeit with some overlapping features), ranging from intellectual disability to amyotrophic lateral sclerosis, but excluding the sympathetic, sensory and optic neuropathies exhibited by FD patients, may indicate variation in function of the Elp subunits independent of their joint actions as part of the Elongator complex.