Mean (standard error) data for: (A) predose CSF nusinersen concentrations (ng/mL; CSF‐evaluable population), (B) change from baseline in HINE‐2 total milestones, and (C) HINE‐2 total milestones. In A, values considered as outliers were excluded from the analyses and included observed concentrations in day 1 predose samples. Only visits with five or more participants are included. In B and C, only visits with four or more participants are included. For the purposes of comparison, the group that initiated nusinersen in part 2 is graphed from day 0 (first nusinersen dose), and in B the initial baseline HINE‐2 score reset to 0. In all graphs, data at each timepoint are offset to allow visualization of all participant groups. CSF, cerebrospinal fluid; HINE‐2, Hammersmith Infant Neurological Examination Section 2; SE, standard error [Color figure can be viewed at wileyonlinelibrary.com]