The invention relates to a method for detecting any mutation at a predetermined site occurring in a known nucleic acid sequence. The method uses primer extension analysis to detect the mutation. Unlabeled terminator is supplied along with labeled non-terminator in the primer extension reaction to detect whether the first nucleic acid base on the template strand that is directly opposite the nucleic acid base immediately 3' to a primer is a mutant. In the primer extension reaction, the terminator is complementary to the wild-type base on the template strand that is directly opposite the nucleic acid base immediately 3' to the primer. Non-terminators are the other nucleotides and are labeled. When the terminator is incorporated into the primer extension strand, primer extension reaction terminates. Incorporation of a labeled non-terminator in the primer extension strand indicates that a mutation has occurred at the predetermined nucleic acid base site.