First testing of ALN-SNCA in people

Key Inclusion Criteria: 1. Diagnosis of Parkinson's disease according to the Movement Disorder Society (MDS) criteria, as assessed by the investigator, with bradykinesia plus at least one of the other cardinal signs of Parkinson's disease (resting tremor, rigidity), without any other known or suspected cause of Parkinsonism 2. A diagnosis of Parkinson's disease for 4 years or less at the screening visit 3. Participant must meet one of the following criteria: 1. Currently not receiving any standard-of-care (SoC) therapy for Parkinson's disease, has not been on oral dopaminergic therapy (ie, levodopa, dopamine agonists, or Monoamine Oxidase B \[MAO-B\] inhibitors) prior to dosing, and is not anticipated to require SoC therapy for Parkinson's disease within approximately 6 months following dosing, or 2. Has been on a stable regimen of oral dopaminergic therapy for at least 3 months prior to dosing and is not anticipated to require dose adjustments within approximately 6 months following dosing 4. BMI ≤35 kg/m\^2 at time of screening visit Key Exclusion Criteria: 1. Medical history indicating a Parkinsonian syndrome other than Parkinson's disease, as defined in the protocol 2. Clinically significant cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, hematological, psychiatric or neurological disease (excluding Parkinson's disease) that, as assessed by the investigator, may confound the results of the study or pose an additional risk to the participant during study participation 3. Medical history of brain or spinal disease/injury that would interfere with the Lumbar Puncture (LP) procedure or CSF circulation, as defined in the protocol 4. Any contraindications to undergo a brain Magnetic Resonance Imaging (MRI) 5. An established allergy or intolerance to lidocaine anesthetic, as defined in the protocol 6. History of intolerance to Intrathecal (IT) injection(s) 7. Current history of bleeding diatheses that would increase risk of bleeding upon LP 8. Has undergone gene therapy, cell therapy or surgical treatment, including deep brain stimulation, for Parkinson's disease NOTE: Other Protocol-defined Inclusion/Exclusion Criteria Apply