Sensitivity of the PCR-based assay in detecting microsatellite alterations. Two DNAs from healthy donors (A and B) showing a different allele for the D18S61 marker (arrowed) were mixed at the indicated percent to obtain a constant final amount of 100 ng DNA as a PCR template. A content of 3% of the donor B DNA showing the lower allele is required for a faint band to appear in the autoradiography. This corresponds to a sensitivity of about 1.5% novel allele DNA in a given sample under these PCR conditions.