TWNK and nervous system disorder: Despite emerging evidence that RRM2B mutations are the third most common cause of multiple mitochondrial DNA deletions in adults, following POLG and PEO1 (now known as C10ORF2) (Fratter et al., 2011; Pitceathly et al., 2011), and RRM2B dysfunction being an important cause of neurological disease, the adult phenotypic and molecular spectrum is not fully understood.