This is a long-term safety and efficacy study in subjects with CLN6 Batten disease who previously received a single intrathecal administration of AT-GTX-501. The assessments described in this long-term follow-up (LTFU) study (AT GTX 501 02) are performed following and in addition to the initial 2 years of assessments in the treatment study (AT-GTX-501-01). In this LTFU study, subjects complete safety and efficacy assessments throughout the study's 13-year duration. Combining the duration of the initial treatment study and this LTFU study, the overall duration reflects a follow-up period up-to 15 years since gene transfer via AT-GTX-501.

The primary outcome for this study is to assess the long-term safety of AT-GTX-501 in subjects with CLN6 Batten disease.

The secondary outcome measure of this study is to assess the long-term efficacy of AT-GTX-501 in subjects with CLN6 Batten disease.