The article describes the development and application of a chemically modified phage T7 DNA polymerase for DNA sequencing by the chain-termination method. The modified enzyme, composed of the 84-kDa T7 gene 5 protein and the 12-kDa E. coli thioredoxin, retains high processivity and a rapid synthesis rate, eliminating the 3' to 5' exonuclease activity. This modification allows for efficient use of nucleotide analogs, such as dideoxynucleotides (ddNTPs), which improve the resolution of gels. The modified enzyme also eliminates band compressions caused by secondary structures, leading to uniform radioactive intensity in the resulting fragments. The procedure involves two steps: a labeling reaction to radioactively label the primer, followed by a termination reaction using ddNTPs. The modified T7 DNA polymerase is shown to be highly processive, lacking exonuclease activity, and efficient in incorporating ddNTPs, making it ideal for DNA sequencing and other applications such as probe preparation and enzymatic amplification of large DNA fragments.The article describes the development and application of a chemically modified phage T7 DNA polymerase for DNA sequencing by the chain-termination method. The modified enzyme, composed of the 84-kDa T7 gene 5 protein and the 12-kDa E. coli thioredoxin, retains high processivity and a rapid synthesis rate, eliminating the 3' to 5' exonuclease activity. This modification allows for efficient use of nucleotide analogs, such as dideoxynucleotides (ddNTPs), which improve the resolution of gels. The modified enzyme also eliminates band compressions caused by secondary structures, leading to uniform radioactive intensity in the resulting fragments. The procedure involves two steps: a labeling reaction to radioactively label the primer, followed by a termination reaction using ddNTPs. The modified T7 DNA polymerase is shown to be highly processive, lacking exonuclease activity, and efficient in incorporating ddNTPs, making it ideal for DNA sequencing and other applications such as probe preparation and enzymatic amplification of large DNA fragments.