SUCLA2 and Mitochondrial encephalopathy: The identification of an ES cell clone with a gene trap of Sucla2 (Fig. 1C) validates the screening strategy given that SUCLA2 is a known mitochondrial disease gene that causes mitochondrial encephalopathy with mtDNA depletion (Elpeleg et al., 2005; Carrozzo et al., 2007; Ostergaard et al., 2007b).