Phenylbutyrate for Monogenetic Developmental and Epileptic Encephalopathy

\###### STXBP1 / SLC6A1 ARM Inclusion Criteria: * Diagnosed with STXBP1-E or SLC6A1-NDD; confirmed by laboratory report (i.e., a genetic test with a pathogenic or likely pathogenic mutation of STXBP1 or SLC6A1-NDD and a clinical picture consistent with the disorder, as determined by the Investigator). Patients with the appropriate clinical picture, a de novo variant of uncertain significance in STXBP1 or SLC6A1-NDD will also be eligible for enrollment, at the discretion of the Investigator. * Is between 2 months and 17 years of age, inclusive. * For children with STXBP1-E, the child must have had at least one seizure in the past 30 days prior to enrollment. If there is high demand for the study and we have several subjects to choose, we will prefer to enroll children with a high number of seizures in the past month. * For SLC6A1-NDD, seizures occur later in the course (typically middle of 1st decade) and so seizures will not be an entry criteria. * Is in general good health, aside from neurological consequences of STXBP1-E or SLC6A1-NDD, as determined by having no concurrent medical illness, in the opinion of the site investigator, that places the subject at increased risk of adverse drug reactions or that will interfere with study follow-up. * Has normal laboratory test results (≤ 1.5 × upper limit of normal \[ULN\]) for serum aminotransferase (aspartate aminotransferas \[AST\] and alanine aminotransferase \[ALT\]) concentrations and ammonia at Screening. * Has normal rena…