My wife was diagnosed with ALS in October, 2017 at the Mayo (FL); diagnosis was confirmed in November, 2017 at Johns Hopkins.
Upon deciding to know, she was notified that she had the C9 mutation in January, 2018 (again, confirmed by Johns Hopkins).
The Mayo and JH offered our 4 adult children (3 live in northern VA and 1 in FL) genetic testing and counseling (outside of a formal research study) at no charge and the option to know or not-know the results of testing. All 4 gave blood samples and 1 additionally gave 2 rounds of spinal fluid (taps) to the Mayo - the Mayo paid for all travel expenses from northern VA for the spinal taps.
I do not know and have not asked my children if they opted to find out the results of this testing. I did advise them beforehand that if it were me, I would opt to not-know. A positive result could weigh on them, psychologically, for years (with no symptoms); whereas, if they didn't know, but started to develop symptoms down-the-road, they would have the option of getting re-tested and know for sure.
It should be stated that all 4 already had children, so the decision to know or not-know was not impacted by fear of potentially passing the C9 mutation to their children.
My wife passed away, from ALS, in June, 2020.
Hope this helps.