The invention relates to a method of synthesizing polydeoxyribonucleotides which comprises causing a thermostable deoxyribunucleotide polymerase to act on deoxyribonucleotides without using any template and any primers, to the thus-obtained polydeoxyribonucleotides, to a method of screening cDNA libraries using the same as probes, to a method comprising joining the DNA resulting from the above method of synthesis to the 3'--OH terminus of a double-stranded DNA containing a gene derived from cells of a living organism, followed by transfection of such cells with the product of joining and culture of the cells, to thereby cause insertion of the product into chromosomal DNA and production of the protein encoded by the gene, to a method of synthesizing chromosomal DNA and to a method of synthesizing chromosomes. According to the invention, it is possible to create DNA not occurring in the nature and create novel proteins using the thus created genes, hence means for providing novel physiologically or pharmacologically active substances can be provided.