| Feature | Observed finding | Available denominator | Interpretation |
|---|---:|---:|---|
| Developmental delay | 21/23 (91.3%) | 23/27 | Ascertainment-selected; data missing for 4 individuals |
| Intellectual disability | 16/16 (100%) | 16 age-eligible, assessed individuals | Severity ranged from mild to profound; not evaluable or unavailable for the remainder |
| Seizure onset before 24 months | 14/18 (77.8%) | 18 individuals with onset reported | Early onset was common among individuals with available data |
| Mean seizure-onset age | 24.6 ± 8.0 months | 18 individuals with onset reported | Summary estimate from available observations; not the entire cohort |
| Generalized tonic–clonic seizures | 12/19 (63.2%) | 19 individuals with seizure-type data | Seizure types were non-exclusive |
| Focal seizures | 7/19 (36.8%) | 19 individuals with seizure-type data | Seizure types were non-exclusive |
| Atonic seizures | 6/19 (31.6%) | 19 individuals with seizure-type data | Seizure types were non-exclusive |
| Myoclonic seizures | 5/19 (26.3%) | 19 individuals with seizure-type data | Seizure types were non-exclusive |
| Abnormal brain MRI | 13/21 (61.9%) | 21 individuals with MRI data | Findings included callosal/cerebellar-vermian abnormalities and delayed myelination |
| Cohort-level caveat | 27 heterozygous point-variant cases | — | Frequencies are descriptive of a clinically ascertained series and must not be extrapolated to all ATP6V0C variant carriers (pqac-00000011) |


*Table: Observed findings in the ascertainment-selected 27-person Mattison ATP6V0C point-variant cohort; denominators vary because clinical data were incomplete. The separate Tian 2022 familial cohort—six individuals, all with febrile-seizure onset at 7–8 months and normal development—is not pooled here (pqac-00000025).*