• A participant is eligible to be included in the study only if all of the following criteria apply:

• Has biallelic CLN2 mutations.

• Has decreased leukocyte TPP1 activity.

• Has clinical signs or symptoms consistent with CLN2 disease (eg, developmental delay, developmental decline, seizure, vision loss, or other signs/symptoms) OR an older sibling with confirmed CLN2 diagnosis.

• Is currently receiving biweekly ICV ERT treatment with cerliponase alfa.

• Meets the following baseline disease condition according to age and CRT as assessed by SD-OCT and confirmed by CRC:

• Participants in the phase of accelerated decline in CRT:

⁃ CRT at baseline ≤210 μm and

⁃ CRT at baseline ≥140 μm in both eyes and

⁃ Age ≤84 months,

∙ Is willing to adhere to the protocol and 5-year visit schedule.

‣ Sexually active female participants of childbearing potential (following menarche) or fertile male participants (following puberty) must be willing to use a medically accepted form of contraception from Screening Visit 2 until 6 weeks after vector administration.

• OR

• Was previously administered TTX-381.

• Upon retrospective review, met the above criteria at the time of administration of TTX-381. IDMC may consider exceptions to this when weighing whether to retrospectively enroll a participant who has received TTX-381.

• Has been recommended for enrollment into the clinical trial by IDMC