(A) A radioactively labeled, complementary primer (red) is annealed to mt-tRNALeu(CUN) and subjected to a RT-PEx reaction. The presence of m1G37 results in RT-PEx pausing, producing a shorter product (dark blue). However, in the absence of m1G37, the extension is able to progress until stalling due to the lack of a dNTP (dTTP in the above case, light blue), producing a longer product.