Trial-Ready Cohort-Down Syndrome (TRC-DS)

Inclusion Criteria: 1. Diagnosis of DS (including trisomy 21, mosaic trisomy 21, Robertsonian translocation trisomy 21 or partial trisomy 21) (as confirmed by genetic testing or medical record review) 2. Provision of signed and dated informed consent form; this includes adults with DS who can provide consent, or for whom an LAR provides consent on behalf of the individual to participate. Adults with DS who cannot consent must sign and date an assent accompanied with a signed and dated consent by legally authorized representative (LAR). 3. Stated availability and willingness to comply with all study procedures and availability for the duration of the study or until referred to a clinical trial 4. Male or female, aged 25-55 inclusive 5. In good general health as evidenced by medical history with no diagnosis of dementia 6. Permitted CNS-active medications, stable in dose for at least 4 weeks or longer. If new medications have been started, medical monitoring team will review on case by case basis to recommend timing of baseline cognitive testing 7. Adequate visual and auditory acuity to allow neuropsychological testing 8. Mental Age of 4 years or greater (based upon the Kaufman Brief Intelligence Test, Second Edition, KBIT-2, verbal age equivalent, or based upon medical records) 9. Ability to complete KBIT-2 with IQ equal to or greater than 40 10. Must speak English or Spanish fluently 11. Must have a reliable Study Partner (may be caregiver, sibling, parent) who is capable of providing correct information about the participant's clinical symptoms and history Exclusion Criteria: 1. Any significant disease or unstable medical condition that could affect participation (i.e., unstable psychiatric disease, unstable cardiac problems, chronic renal failure, chronic hepatic disease, severe pulmonary disease) 2. Participants in whom magnetic resonance imaging (MRI) is contraindicated including, but not limited to, those with a non-compatible pacemaker, presence of MRI-incompatible metallic fragments near the eyes or spinal cord, or cochlear implant (Dental fillings do not present a risk for MRI) 3. Participants unable to complete MRI procedure 4. History, within the last 5 years of a primary or recurrent malignant disease with the exception of non-melanoma skin cancers, resected cutaneous squamous cell carcinoma in situ, basal cell carcinoma, cervical carcinoma in situ, or in situ prostate cancer with normal prostate-specific antigen post-treatment 5. Clinically significant abnormalities in B12 or TFTs that might interfere with the study. A low B12 is exclusionary, unless follow-up labs (homocysteine (HC) and methylmalonic acid (MMA)) indicate that it is not physiologically significant. A high TSH is exclusionary unless follow up T3/T4 levels indicate that it is not physiologically significant. 6. Clinically significant abnormalities in screening laboratories 7. For participants undergoing CSF collection: a current blood clotting or bleeding disorder, or significantly abnormal PT or PTT at screening or if on anti-coagulation therapy (e.g. warfarin) 8. Concurrent participation in a clinical trial for an investigational product or concurrent participation in longitudinal study with overlapping outcome measures/procedures is prohibited with the exception of ABC-DS co-enrollment or as approved by project director 9. Participants whom the investigator deems to be otherwise ineligible. The Investigators should consult with the Coordinating Center on any issues that may disqualify the participant from participation in future clinical trials to determine whether enrollment into TRC-DS would be appropriate