Phase 1/2 Study of KRRO-121 in Healthy Volunteers and Patients With UCD

Inclusion Criteria: * Adult male or female participants, 18 to 65 years of age * Healthy as determined by medical history, physical examination, vital signs, 12-lead electrocardiogram (ECG), and screening laboratory assessments, in the opinion of the Investigator. * Female participant of childbearing potential or male participant that is willing to use an approved, reliable means of contraception * Willing and able to provide signed written informed consent and to comply with all study requirements Exclusion Criteria: * Any medical history that is clinically significant in the opinion of the Principal Investigator and Sponsor Medical Monitor * Any laboratory value at screening or predose on Day -1 outside the local laboratory reference range that is clinically significant in the opinion of the Principal Investigator and Sponsor Medical Monitor * Evidence of active systemic infection at Screening or Day 1 requiring antimicrobial, antiviral, or antifungal therapy * Use of any investigational drug or device within 3 months or 5 half-lives (whichever is longer) prior to Day 1 * Concurrent participation in any other interventional clinical study