Somatic, brain-restricted variants in SLC35A2 were first identified by Winawer et al., who reported mosaic pathogenic mutations confined to resected cortical tissue from patients with pharmacoresistant focal epilepsy, absent in blood-derived DNA, thus establishing the concept of brain-only SLC35A2 mosaicism [33] (Figure 3 and Figure 4). Here, SLC35A2 is linked to focal epilepsy.