In order to further evaluate the specificity and sensitivity of eIF2B GEF activity in patients' LLB regarding the diagnosis of eIF2B-related disorders, we extended this initial cohort of 30 to 63 mutated patients and compared the results not only to healthy non mutated subjects but also to patients with defined leukodystrophies or CACH/VWM-like diseases. The gene discussed is EIF2B1; the disease is leukodystrophy.