Genetic causes were identified in cases with clear clinical evidence for a monogenic disease in which routine diagnostics remained unremarkable, e.g., a translocation concerning the FBN1 gene in Marfan syndrome, a mosaic deletion of the CDKL5 gene associated with developmental and epileptic encephalopathy 2, and an inversion of the NF1 gene in neurofibromatosis type 1 [32,33,34]. This evidence concerns the gene CDKL5 and early-infantile DEE.