This study aims to understand how differences in the NOTCH2NLC gene affect the symptoms and course of neuronal intranuclear inclusion disease (NIID), a rare inherited neurological disorder. NIID is caused by an abnormal expansion of a GGC DNA repeat in the NOTCH2NLC gene, but members of the same family can have very different repeat sizes and patterns, leading to a wide variety of problems-such as difficulties with memory, movement, sensation, or involuntary body functions. The main goal is to uncover how these genetic differences (repeat length and interruption pattern) contribute to the severity and type of symptoms.

The study is being conducted at Sichuan Provincial People's Hospital and will enroll approximately 12 individuals from a single family, including those diagnosed with NIID, family members who carry the genetic change but are not yet sick, and healthy relatives. Participants must be 18-85 years old, able to complete genetic testing and a small skin biopsy, and willing to provide informed consent. Those who are medically unstable or otherwise unable to participate will not be enrolled.

The study has both a retrospective part (collecting past medical records) and a prospective follow-up. At the beginning, all participants will have a physical exam, provide a blood sample (for long-read DNA sequencing and RNA sequencing), and undergo a 3-mm skin biopsy to look for disease-related protein deposits. Brain MRI and nerve/muscle electrical tests will also be performed if not done recently. After this baseline visit, everyone will be followed every 6 months for a total of 2 years (5 visits total). Each follow-up visit includes assessments of thinking, memory, movement, autonomic function, pain, and quality of life, along with a neurological exam and repeat imaging/electrical tests as needed. At the final 24-month visit, another blood sample will be taken for RNA sequencing to see how gene activity changes over time.

This is an observational study; there is no experimental treatment. Participants will be compensated a total of ¥3,000 across all visits for their time and travel. All data and samples will stay in China and will not be shared internationally.

Inclusion Criteria:

• Member of a single family (pedigree) with known NOTCH2NLC-related neuronal intranuclear inclusion disease (NIID), including clinically diagnosed patients, asymptomatic GGC repeat expansion carriers, and healthy relatives without the expansion.
• Age 18 to 85 years at the time of enrollment.
• Able and willing to undergo genetic testing for NOTCH2NLC (including long-read sequencing) and a skin punch biopsy.
• Able to provide written informed consent.

Exclusion Criteria:

• Unstable vital signs or any acute medical condition that would interfere with study participation.
• Any condition that, in the opinion of the investigator, makes the participant unsuitable for the study.

This study will enroll approximately 12 participants from a single family (pedigree) with NOTCH2NLC-related neuronal intranuclear inclusion disease (NIID). The study population comprises three categories of family members: individuals with a clinical diagnosis of NIID, asymptomatic carriers of the NOTCH2NLC GGC repeat expansion, and healthy relatives who do not carry the expansion. All participants are adults aged 18 to 85 years recruited from the Health Management Center of Sichuan Provincial People's Hospital in China. Given the rarity and genetic nature of the disease, this single-family design is intended to control for shared genetic background and environmental factors while examining the effect of different GGC repeat characteristics on clinical phenotype.