The purpose of this study is to gather information on the possible symptoms that patients with atypical neuronal ceroid lipofuscinosis type 2 (also known as aTPP1 or atypical tripeptidyl peptidase deficiency) have and how they change over time.

This study aims characterize the natural history of atypical TPP1 deficiency patients via longitudinal multidisciplinary assessments.

Multifaceted clinical, laboratory, imaging, and diagnostic assessments will be performed at regular intervals upon enrolled aTPP1 deficiency patients, collated, and analyzed over a three-year longitudinal period.

Inclusion Criteria:

• Any patient with documented TPP1 enzymatic deficiency or TPP1 sequence variants
• Onset of first symptom after 4 years of age
• Parental provision of informed consent; child provision of assent (if necessary)

Exclusion Criteria:

• Any patient with "Classical" TPP1 deficiency (onset of first symptom prior to 4 years of age)
• Investigator assessment that patient is not suitable candidate to participate in the study

Any patient with documented TPP1 enzymatic deficiency or TPP1 sequence variants with onset of first symptom after 4 years of age