Analysis of this extended cohort showed that eIF2B GEF activity measured in patients' LLB distinguishes eIF2B-mutated patients from those with eIF2B-unrelated leukodystrophies with 100% positive predictive value (PPV) and 89% negative predictive value at ≤77.5% threshold. The gene discussed is EIF2B1; the disease is leukodystrophy.