A Study to Evaluate the Efficacy, Safety, Pharmacokinetics and Pharmacodynamics of ION363 in Amyotrophic Lateral Sclerosis Participants With Fused in Sarcoma Mutations (FUS-ALS)

Inclusion Criteria for Part 1:

   1. Participants in:

   Cohort A must be 12 - 65 years of age with signs or symptoms consistent with an ALS
   disease. If 30 to 65 years of age, have an ALSFRS-R pre-study slope ≥ 0.4 points per
   month (calculated as [48-Screening ALSFRS-R score]/time in months since symptom onset)
   Cohort B must be> 30 years of age, with signs or symptoms consistent with an ALS
   disease process and have an ALSFRS-R pre-study slope < 0.4 points per month
   (calculated as [48-Screening ALSFRS-R score]/time in months since symptom onset). If
   between the ages of 30 - 65 years, inclusive, or > 65 years of age with no ALSFRS-R
   pre-study slope criterion

   2. Confirmed genetic mutation in FUS in a clinical laboratory improvement amendments
   (CLIA) certified, CE-marked, or equivalent testing laboratory. Mutations must be
   reviewed and approved by a variant classification committee.

   3. Upright (sitting position) slow vital capacity (SVC) as adjusted for sex, age, and
   height ≥ 50 percent (%) of predicted value

   4. Participants taking edaravone must be on a stable dose for ≥ 28 days prior to
   Screening and riluzole must be on a stable dose for ≥ 28 days prior to Day 1, and
   willing to continue on that dose throughout the duration of the study, unless the
   Investigator determines that it should be discontinued for medical reasons, in which
   case it may not be restarted during the study

   5. Stable accompanied medications and nutritional support for at least 1 month prior to
   Study Day 1. Accompanied medications or nutritional support that have not been stable
   for at least 1 month prior to Study Day 1 may be allowed in consultation with the
   Sponsor Medical Monitor or designee.

   6. Has an informant/caregiver who, in the Investigator's judgment, has frequent and
   sufficient contact with the participant as to be able to provide accurate information
   about the participant's cognitive and functional abilities at Screening. Participants
   < 18 years old at Screening must have a trial partner (parent, caregiver or other) who
   is reliable, competent and at least 18 years of age, is willing to accompany the
   participant to trial visits and to be available to the Study Center by phone if
   needed, and who (in the opinion of the Investigator) is and will remain sufficiently
   knowledgeable of participant's ongoing condition to respond to Study Center inquiries
   about the participant

Inclusion Criteria for Part 2:

   1. Completed, or rescued from, Part 1, or

   2. Enrolled and received at least 1 dose of ION363 in the Investigator-initiated EAP
   program

   3. Patient meeting Criteria #1-2 is otherwise suitable for study participation, in the
   opinion of the Investigator

Exclusion Criteria for Part 1:

   1. Requiring permanent ventilation (> 22 hours of mechanical ventilation [invasive or
   noninvasive] per day for > 21 consecutive days) and/or tracheostomy

   2. Any known ALS-associated mutations except FUS

   3. Positive test result for:

      1. Human immunodeficiency virus (HIV)

      2. Hepatitis C (HCV), unless previously treated and has been serum/plasma HCV RNA
      negative for at least 6 months after the end of treatment

      3. Hepatitis B (HBV) by HBV surface antigen test, unless currently on
      nucleotide/nucleoside analogue treatment

   4. Clinically significant (CS) abnormalities in medical history (e.g., previous acute
   coronary syndrome within 3 months of Screening, major surgery within 2 months of
   Screening) or physical examination

   5. Uncontrolled hypertension (blood pressure [BP] > 160/100 millimeters of mercury [mm
   Hg])

   6. Malignancy within 1 year of Screening, except for basal or squamous cell carcinoma of
   the skin or carcinoma in situ of the cervix that has been successfully treated.
   Participants with a history of other malignancies that have been treated with curative
   intent and which have no recurrence within 6 months may also be eligible per
   Investigator judgement.

   7. Obstructive hydrocephalus

   8. Known significant brain or spinal disease that would interfere with the lumbar
   puncture (LP) process, CSF circulation or safety assessment, including tumors or
   abnormalities by magnetic resonance imaging (MRI) or computed tomography (CT),
   subarachnoid hemorrhage, suggestion of raised intracranial pressure on MRI or
   ophthalmic examination, spinal stenosis or curvature, chiari malformation, obstructive
   hydrocephalus, syringomyelia, tethered spinal cord syndrome and connective tissue
   disorders such as Ehlers-Danlos syndrome and Marfan syndrome

   9. Concurrent participation in any other interventional clinical study

10. Previous treatment with an oligonucleotide (including small interfering RNA [siRNA]).
   This exclusion criterion does not apply to COVID-19 vaccinations, which are allowed

11. Treatment with another investigational drug, biological agent, or device, including,
   but not limited to sodium phenylbutyrate, within 1 month of Screening, or 5 half-lives
   of investigational agent, whichever is longer

12. History of gene therapy or cell transplantation or any other experimental brain
   surgery

13. Have any other conditions, which, in the opinion of the Investigator would make the
   participant unsuitable for inclusion, or could interfere with the individual
   participating in or completing the study