GFER and hyperinsulinemic hypoglycemia, familial, 4: Mutations affecting GFER (full name: growth factor homolog to yeast ERV1 responsible for liver regeneration in humans) (Di Fonzo et al., 2009) and DPP1 (deafness dystonia peptide 1) (Aguirre et al., 2006; Jin et al., 1996), two components of the mitochondrial protein import machinery, were found in individuals presenting with multiple mitochondrial deficiencies and a complex clinical phenotype characterized by visual and/or hearing problems, developmental delay, mental retardation and myopathy.