The investigators will examine a total of 20 patients at six month intervals for up to five years. The investigators will evaluate gastrointestinal function, lean body mass, neuropathy, neuropsychological capability, quality of life, nutrition, motor function and biochemical parameters.
Inclusion Criteria: 1. Thymidine Phosphorylase (TP) defect: 1. homozygous or 2. compound heterozygous mutations in the TYMP gene, and/or 3. TP enzyme activity of <20% of normal. 2. Increased plasma Thd > 3 micromole/L 3. Increased plasma dUrd > 7.5 micromole/L 4. Age requirement of at least 5 years of age. Exclusion Criteria: 1. Participation in an interventional (study medication or other experimental intervention) study (within 1 month of participation in this study). 2. Unable to travel to site for research visits. 3. Unwillingness to sign informed consent form. 4. Substance abuser