This invention concerns a reagent composition comprising at least two different terminators of a nucleic acid template-dependent, primer extension reaction. This invention also concerns a method for determining the identity of a nucleotide base at a specific position in a nucleic acid of interest. This invention further concerns a method for determining the presence or absence of a particular nucleotide sequence in a sample of nucleic acids. This invention further concerns a method for identifying different alleles in a sample containing nucleic acids. This invention further concerns a method for determining the genotype of an organism at one or more particular genetic loci.