A method for determining the sequence of a target nucleic acid, including steps of contacting a target nucleic acid with a polymerase which is associated with a nanopore to sequentially remove nucleotide triphosphates from the target nucleic acid by pyrophosphorolysis; and distinguishing the different base moieties for the nucleotide triphosphates that are removed wherein the base moieties are distinguished by a change in current in the nanopore.