The new detection method described above utilizes the unique nature of low-molecular-weight PEI, i.e., it cannot form an insoluble complex with a single-stranded anionic polymer with a low molecular weight such as an oligo DNA probe, but it can form an insoluble complex with DNA with a high molecular weight such as LAMP product. Until now, not much attention appears to have been paid to the complex consisting of oligo DNA and PEI with a molecular weight of 1,000 or lower, which was used in this study. This is probably because the interaction between low-molecular-weight DNA and oligo DNA is very weak and because its practical utility as a vector or nucleic acid precipitant is low. However, Kunath et al. reported that the complex formed by PEI (5 kDa) and plasmid DNA was more unstable than that formed by PEI (25 kDa) [15] . Osland and Kleppe reported that spemidine, which has a structure similar to that of PEI, formed an aggregate with double-stranded oligo DNA, but it did not form an aggregate with single-stranded DNA [10] . Drawing upon these results, the low-molecular-weight PEI used in this study is thought to interact with DNA with high selectivity to differences in DNA concentration and structure (number of strands and molecular weight). The fact that we took advantage of this nature of lowmolecular-weight PEI as a nucleic acid precipitant for detection led to the establishment of this detection method.