POLG and mitochondrial disease: 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).