Sometime in the mid-1990's, researchers discovered a gene for
familial ALS--the autosomal dominant form of ALS that can be
inherited and passed from generation to generation.
Unfortunately, only about 5% of ALS cases are familial--and this gene
isn't implicated in all familial cases--so this particular discovery
doesn't really help people like bitter_engineer's uncle, Stephen
Hawking, or a friend of my family who died recently. The gene is
thought to produce superoxide dismutase, an enzyme that's involved
in the metabolism of free radicals.
As the disease progresses, motor neurons slowly degenerate, but sensory
neurons are unimpaired, meaning that the patient can still feel pain,
discomfort, heat, cold, etc. Bowel and bladder function are
preserved, as are most autonomic functions; so are mental abilities (as
bitter_engineer says). The patient often dies from a problem with
swallowing or breathing; pneumonia is common.
ALS belongs with Alzheimer's disease on top of the list of "Most
Horrible Diseases Known to Man."