This protocol has two parts - the Main Study which is 42 days in length and the Treatment Extension which allows the patients who complete the Main Study to continue receiving treatment with uridine triacetate. The purpose of this study is to replace oral administration of uridine with oral administration of uridine triacetate in patients with hereditary orotic aciduria who have received (or would reasonably be expected to receive) clinical benefit from treatment with exogenous uridine. It is also to document the continued clinical benefit of exogenous uridine when patients are switched from oral administration of uridine to oral administration of uridine triacetate.

Data to be collected during the Main Study include demographic, baseline disease information and medical history including all prior disease-directed therapy. In addition, vital signs, laboratory values and adverse events information will be collected and recorded. Urine samples will be obtained and measured for orotic acid and orotidine levels. Systemic levels of uridine will be evaluated from plasma samples collected at set timepoints.

Upon successful completion of the Main Study and entry into the Treatment Extension, physical exams and vital signs will be performed every six (6) months. Additionally, plasma samples to measure systemic levels of uridine and urine samples to measure levels of orotic acid and orotidine will be collected every (6) six months.

Inclusion Criteria (Main Study):

• Patients with diagnosed hereditary orotic aciduria
• Judged by the investigator to have the initiative and means to be compliant with the protocol
• Able to take oral medications
• Able to provide written informed consent (patient or legally authorized representative)
• Females of childbearing potential must have a negative pregnancy test at screening
• Females of childbearing potential or males with partners of childbearing potential are to use one of the following acceptable birth control methods:
• Surgically sterile or partner is surgically sterile
• Using adequate contraception (hormonal contraceptives, double barrier methods, or intra-uterine devices)
• Patients who claim to be sexually inactive agree to use an acceptable method of contraception should she or he become sexually active from 14 days prior to first dosing, throughout the study and for 14 days after the last dose administration

Exclusion Criteria (Main Study):

• Has a known allergy to uridine triacetate or any of its excipients
• Known to have ornithine transcarbamoylase deficiency
• Unable to have the initiative and means to be compliant with the protocol
• Unable to be compliant with taking oral medications
• Unable to provide written informed consent (patient or legally authorized representative)
• Female who is pregnant or lactating

Inclusion Criteria (Treatment Extension)

• Patient successfully completed the Main Study

Exclusion Criteria (Treatment Extension)

• Patient did not successfully complete the Main Study

Safety data collected for 24 months.