Tail 1 and tail 2 target DMPK isoforms to the ER or mitochondria, respectively. HA-DMPK isoforms were transfected to N2A cells to examine their subcellular localization in more detail. Colocalization with an ER-resident GFP mutant demonstrated localization of DMPK A to the ER (A to C). Similarly, colocalization with cytochrome c oxidase, a mitochondrial protein, showed that DMPK C is confined to mitochondria (D to F). Bars, 10 μm.