TWNK and hearing loss disorder: The presence of significant bulbar weakness, hearing loss and gastrointestinal symptoms should guide clinicians towards RRM2B genetic analysis before POLG and PEO1, given that these key features are more common components of the clinical spectrum (bulbar dysfunction 42% versus 37% and 12%; sensorineural hearing loss 36% versus 11% and 9%; and gastrointestinal symptoms 19% versus unreported and 9% in RRM2B versus POLG and PEO1-related PEO syndromes, respectively) (Horvath et al., 2006; Fratter et al., 2011).