Cerebral folate deficiency is reported in several types of mitochondrial disease (Pineda et al. 2006; Garcia-Cazorla et al. 2008b), including POLG mutations (Hasselmann et al. 2010; Rajakulendran et al. 2016), ranging from mild deficiency to more severe forms that can mimic primary folate disorders, such as those due FOLR1 mutations (Cario et al. 2009). The gene discussed is POLG; the disease is folate deficiency.