SCN2A and biotinidase deficiency: Patient B was investigated at 9 months of age using a gene panel for leukodystrophies (Table EV1B), which revealed a homozygous BTD variant responsible for the biotinidase deficiency (c.476G>A, p.Arg159His) as well as a homozygous variant of unknown significance in SCN2A (c.5995G>T, p.Asp1999Tyr), which was not interpreted as being causative (see Discussion).