Phase III Long-Term Extension Study With Dexpramipexole

Inclusion Criteria: 1. Participants who completed the double-blind study intervention treatment period during either of the Phase III studies EXHALE-2 or EXHALE-3. 2. Participants capable of giving signed informed consent/assent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. 3. Negative urine pregnancy test for women of childbearing potential (WOCBP; after menarche) at the Baseline Visit/Day 1. 4. WOCBP (after menarche) must use either of the following methods of birth control, from the Baseline Visit/Day 1 through the End of Study Visit: 1. A highly effective form of birth control (confirmed by the investigator). Highly effective forms of birth control include: true sexual abstinence, a vasectomized sexual partner, Implanon, female sterilization by tubal occlusion, any effective intrauterine device (IUD), IUD/intrauterine system, levonorgestrel intrauterine system, or oral contraceptive. -OR- 2. Two protocol acceptable methods of contraception in tandem. Women not of childbearing potential are defined as women who are either permanently sterilized (hysterectomy, bilateral oophorectomy, or bilateral salpingectomy), or who are postmenopausal. Women will be considered postmenopausal if they have been amenorrheic for ≥12 months prior to the planned date of the Baseline Visit/Day 1 without an alternative medical cause. The following age specific requirements apply: 3. Women \<50 years old will be considered postmenopausal if they have been amenorrheic for 12 months or more following cessation of exogenous hormonal treatment and follicle stimulating hormone levels in the postmenopausal range. 4. Women ≥50 years old will be considered postmenopausal if they have been amenorrheic for 12 months or more following cessation of all exogenous hormonal treatment. Exclusion Criteria: 1. Clinically significant change in health status during either of the Phase III studies EXHALE-2 or EXHALE-3 which in the opinion of the investigator would make the participant unsuitable for participation in this study. 2. Participants with allergy/intolerance to dexpramipexole or any of its excipients. 3. Participants who met permanent discontinuation criteria or permanently discontinued study treatment for any reason in either of the Phase III studies EXHALE-2 or EXHALE-3.. 4. Treatment with selected drugs known to have a substantial risk of neutropenia in the past 30 days. 5. Pregnant or breastfeeding women. 6. Males who are unwilling to use an acceptable method of birth control during the entire study period (ie, condom with spermicide).