Epilepsy before 5 years of age was statistically significant among groups of patients (p value 0.0001 MECP2 vs. CDKL5 and p value < 0.044 MECP2 vs. FOXG1), since it was present in 63% of MECP2-mutated patients, in 96.9% (31 out of 32) of CDKL5 cases, and in 87.5% of FOXG1-mutated patients. Here, CDKL5 is linked to epilepsy.