ABCD1 and X-linked adrenoleukodystrophy: The visual impairment in X-ALD patients are caused by two mechanisms: 1) the loss of color vision in a subgroup of X-ALD patients are caused by continuous gene deletions in X chromosome where ABCD1 gene and red pigment gene are in close proximity (Sack et al., 1989) or by chromosome rearrangements (Alpern et al., 1993), 2) the cortical blindness associated with X-ALD is caused by visual track demyelination, which can subsequently lead to retrograde degeneration of ganglion cells (Kaplan et al., 1995; Ohkuma et al., 2014).