precancerous lesions, and subject having precancerous lesions and being treated with an anti-cancer agent; (2) isolating total microbial genomic DNA from the fecal samples to provide total microbial genomic DNA; (3) comparing the total microbial genomic DNA using fingerprint spectrum analysis; (4) identifying key fingerprint bands correlated with the effect of the anti-cancer agent; (5) identifying key microorganisms associated with the key fingerprint bands; (6) designing microbial sequence-specific primers and probes; and (7) determining the quantitative differences of the key microorganisms in fecal samples to identify an indicator microorganism for monitoring the effect of the anti-cancer agent.