The invention concerns the introduction of predetermined genetic changes in target genes of a living cell by introducing an oligodeoxynucleotide encoding the predetermined change. The oligodeoxynucleotides are effective in animal, plant and bacterial cells. Specific end modifications that greatly increase the effectiveness of the oligodeoxynucleotides in bacteria are described. Surprisingly, unmodified oligodeoxynucleotides can be as effective in mammalian cells, including in vivo hepatocytes, as the modified nucleotides and can be as effective or more effective than chimeric oligonucleotides that consist of a mixture of deoxynucleotides and 2'-O-methyl ribonucleotides.