The primary purpose of this study is to evaluate the safety and tolerability of ION440.
Males aged ≥ 2 to ≤ 65 years, depending on specific cohort and group, at the time of informed consent.
Group A: ≥ 8 to ≤ 65 years old
Group B: 2 to 7 years old, inclusive
Participant has at least one parent or caregiver ≥ 18 years old capable of providing informed consent and able to comply with all study requirements and activities.
Documented diagnosis of severe MECP2 duplications including terminal duplication and/or translocation or MECP2 triplication OR clinical features associated with severe variant structure including (a) onset of seizures prior to age 5 (for those aged 5 and above at signing of ICF), (b) oxygen dependence, (c) microcephaly, IF MECP2 genetic structure information is unavailable.
Clinically significant vital sign or ECG abnormality at Screening
Known brain or spinal disease that would interfere with the LP procedure, or CSF circulation or presence of other factors would affect the safety of the LP procedure.
Has any concomitant disease or condition or circumstance, or any finding at Screening that, in the opinion of the Investigator, makes the participant unsuitable for enrollment or that could interfere with the conduct of the study or that would pose an unacceptable risk to the participant in this study.