• Signed informed consent by study participant or parent(s) or legally authorized representative (LAR) and signed informed assent by the study participant (when applicable)
• Aged 2.5 to 10 years (inclusive) at study entry
• Diagnosis of ACH
• Ambulatory and able to stand without assistance
• Study participants and parent(s) or LAR(s) are willing and able to comply with study visits and study procedures
• Have hypochondroplasia or short stature condition other than ACH (e.g trisomy 21, pseudoachondroplasia, psychosocial short stature)
• In females, having had their menarche
• Height < -2 or > +2 standard deviations for age and sex based on reference tables on growth in children with ACH
• Annualized height growth velocity ≤1.5 cm/year over a period ≥6 months prior to screening
• Have a concurrent disease or condition that in the view of the Investigator and/or Sponsor, may impact growth or where the treatment is known to impact growth.
• Significant abnormality in screening laboratory results.
• Have been treated with growth hormone, insulin-like growth factor 1 (IGF 1), or anabolic steroids in the previous 6 months or long-term treatment (>3 months) at any time
• Have received a C-type natriuretic peptide (CNP) analog or treatment targeting fibroblast growth factor receptor (FGFR) inhibition at any time
• Have had regular long-term treatment (>1 month) with oral corticosteroids (low-dose ongoing inhaled steroid for asthma is acceptable)
• Have used any other investigational product or investigational medical device for the treatment of ACH or short stature
• Have had previous limb-lengthening surgery

Key Part A, B and C:
•Genetic documentation of 5q SMA (homozygous gene deletion, mutation, or compound heterozygote) Part A:
• Onset of clinical signs and symptoms consistent with SMA at > 6 months (> 180 days) of age (i.e., later-onset SMA)
• Age 2 to ≤ 15 years, inclusive, at the time of informed consent Part B:
• Participants with SMA symptom onset ≤ 6 months (≤ 180 days) of age (infantile onset) should have age > 1 week to ≤ 7 months (≤ 210 days) at the time of informed consent
• Participants with SMA symptom onset > 6 months (> 180 days) of age (later onset):
• Age 2 to < 10 years at the time of informed consent
• Can sit independently but has never had the ability to walk independently
• HFMSE score ≥ 10 and ≤ 54 at Screening Part C:
•Currently on nusinersen treatment at the time of Screening, with the first dose being at least 1 year prior to Screening Part C Cohort 1:
•Participants of any age (individuals ≥18 years of age at Screening must be ambulatory) Part C Cohort 2:
• Participants ≥18 years of age at Screening (can be ambulatory or nonambulatory)
• HFMSE total score ≥4 points at Screening
• RULM entry item A score ≥3 points at Screening Key Part A, B and C:
• Presence of an untreated or inadequately treated active infection requiring systemic antiviral or antimicrobial therapy at any time during the Screening period
• Presence of an implanted shunt for the drainage of cerebrospinal fluid (CSF) or of an implanted central nervous system (CNS) catheter
• Hospitalization for surgery, pulmonary event, or nutritional support within 2 months prior to Screening or planned within 12 months after the participant's first dose Part A:
• Respiratory insufficiency, defined by the medical necessity for invasive or noninvasive ventilation for > 6 hours during a 24-hour period, at Screening
• Medical necessity for a gastric feeding tube
• Treatment with an investigational drug given for the treatment of SMA, biological agent, or device within 30 days or 5 half-lives of the agent, whichever is longer, prior to Screening or anytime during the study; any prior or current treatment with any survival motor neuron-2 gene (SMN2)-splicing modifier or gene therapy; or prior antisense oligonucleotide treatment, or cell transplantation Part B:
• Treatment with an investigational drug including but not limited to the treatment of SMA, biological agent, or device within 30 days or 5 half-lives of the agent, whichever is longer, prior to Screening or anytime during the study; any prior or current treatment with any SMN2-splicing modifier or gene therapy; or prior antisense oligonucleotide treatment, or cell transplantation
• Participants with SMA symptom onset > 6 months (> 180 days) of age (later onset):
• Respiratory insufficiency, defined by the medical necessity for invasive or noninvasive ventilation for > 6 hours during a 24-hour period, at Screening
• Medical necessity for a gastric feeding tube
• Participants with SMA symptom onset ≤ 6 months (≤ 180 days) of age (infantile onset): Signs or symptoms of SMA present at birth or within the first week after birth Part C:
• Concurrent or previous participation and/or administration of nusinersen in another clinical study
• Concomitant or previous administration of any SMN2-splicing modifier (excluding nusinersen) or gene therapy, either in a clinical study or as part of medical care.
• Concurrent or previous participation in any interventional investigational study for any other drug or device within 30 days or 5 half-lives of the agent, whichever is longer, prior to Screening NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.