Alexander disease is a rare disorder of cerebral white matter due to a dysfunction of astrocytes. The most common infantile form presents as a megalencephalic leukodystrophy. Mutations of the GFAP gene, encoding Glial Fibrillary Acidic Protein, have been recognized as the cause of Alexander disease. Glial Fibrillary Acidic Protein is the major intermediate filament protein in astrocytes, its functional rod domain is conserved in sequence and structure among other intermediate filament proteins. We report here two cases of infantile Alexander disease with early onset and severe course, caused by DE NOVO mutations A364 V and Y366C. Both affected GFAP residues are part of a highly conserved coiled-coil trigger motif in the C-terminal end of segment 2B, probably required for the stability of intermediate filament molecules. Comparable effects are seen with mutations of the corresponding residues of the gene coding for keratin 14, another intermediate filament, this further supports the hypothesis that these positions of the trigger motif are generally critical for a normal function of intermediate filaments.