Cancer can be diagnosed by identifying DNA and microRNA base sequences that have the same base length yet differ in a few base sequences, if the abundance ratios of these slightly deviant base sequences can be determined. However, such quantitative analyses cannot be performed using the current DNA sequencers. Here we determine entire base sequences of four types of DNA corresponding to the let-7 microRNA, which is a 22-base cancer marker. We record the single-molecule conductances of the base molecules using current-tunneling measurements. In addition, we count the numbers of molecules in a solution to determine the abundance ratios of two DNA strands that differ by a single base sequence.