• Diagnosis of Cockayne syndrome (CS), xeroderma pigmentosum (XP), or trichothiodystrophy (TTD), based on genetic testing and/or key clinical characteristics l characteristics

• Has one or more of the following neurodevelopmental or neurological complications

• Gross motor delay (non-ambulatory or started walking after age 18 months)

• Language delay (non-verbal or started talking after 18 months)

• Altered muscle tone (hypertonia, dystonia, hypotonia)

• Gait difficulties, including stiff gait, short stride, frequent falls, use of orthotics, use of walker

• Tremors

• Microcephaly

• Is a family member of an individual with the above condition

• No restrictions regarding current ambulatory status

• Minimum age for enrollment eligibility will be 6 months due to fragility of neonates with severe forms of DNA repair disorders and limitations of motor assessment scales in infants younger than 6 months. There will be no maximum age for enrollment eligibility.

• No restrictions regarding gender, race, or ethnicity.

• Voluntary written consent from the participant if adult capable of consenting or parent/guardian if minor or not capable of consenting

• Written consent of Legally Authorized Representative if enrolling adult lacks capacity to consent