Study of YOLT-203 in Children and Adults With Primary Hyperoxaluria Type 1 (PH1)

Inclusion Criteria * You are 6 years old or older. * You have been diagnosed with PH1 (primary hyperoxaluria type 1) through genetic testing. * Urine tests show that your oxalate levels are not within certain limits (based on at least two 24-hour urine collections). * If you take vitamin B6, your dose has been stable for at least 3 months, and you are willing to keep it the same during the study. * You (and/or your parent or legal guardian, if you are under 18) understand the study and agree to follow all study requirements, including signing a consent form. Exclusion Criteria * You have signs that oxalate has affected parts of your body outside the kidneys. * Your liver blood tests are not within certain limits. * You have certain abnormal blood clotting results (unless controlled with medication like warfarin). * You have HIV, or active or chronic hepatitis C. * You are pregnant, planning to become pregnant, or breastfeeding. * You are not willing to use birth control during the study and for 6 months after (if applicable). * You are unable to limit alcohol intake to no more than about 2 drinks per day. * You have had alcohol or drug abuse in the past year. * You are unable to follow standard treatments for PH1 (such as drinking plenty of fluids, taking prescribed medications like potassium citrate, or following dietary advice).