Valuable clues about how axons degenerate in MS can be gained from axon pathology in other disorders and experimental models. We discuss the similarities in mechanism and morphology of axon pathology in diverse circumstances revealed using mutant mice. The slow Wallerian degeneration mutation, Wld(S), delays three types of axon degeneration previously considered distinct: Wallerian degeneration of injured axons, 'dying-back' of axons in peripheral nervous system disease, and axonal spheroid pathology in gracile axonal dystrophy (gad) mice. Therefore, axon degeneration mechanisms are more uniform than previously thought and, in gad at least, axonal swelling is either related to or a consequence of Wallerian degeneration. Both axonal swelling and the accumulation of amyloid precursor protein through impaired axonal transport are common to MS, gad, and many other CNS disorders, indicating a degree of shared mechanism. YFP-H transgenic mice express YFP in a representative subset of neurons enabling unprecedented imaging of axon morphology and pathology over considerable longitudinal distances. Using this method, we have observed unbroken axons with multiple constrictions and dilatations in VEGF(delta/delta) mice, a model of amyotrophic lateral sclerosis (ALS). Similar morphologies have been described in MS, stroke, and other disorders, again suggesting a uniformity of axon degeneration mechanisms.