Schematic diagram showing potentially toxic RNA and protein products expressed from a repeat expansion mutation through a combination of bidirectional transcription, ATG-initiated and repeat associated non-ATG (RAN) translation. In vitro studies predict ATG-initiated and RAN translation can both occur when the repeat is located in an open reading frame (ORF) [12]. While a single ATG-initiated protein is illustrated, multiple ATG-initiated proteins may be produced if there are multiple ORFs. Additionally, RAN translation of the expanded repeat results in the expression of up to six distinct RAN proteins. For example, a CTG•CAG expansion can produce poly-Gln, poly-Ala and poly-Ser RAN proteins from the CAG transcript and poly-Leu, poly-Ala and poly-Cys RAN proteins from the CUG transcript. Each RAN protein, depending upon flanking sequences, may contain distinct C-terminal regions and an ATG-initiated protein in the same reading frame may also have a distinct N-terminal region.