The applications of synthetic oligodeoxyribonucleotides to problems in molecular biology described in this article are those where the oligodeoxyribonucleotide is a probe for a specific region of a nucleic acid. This includes the isolation of the iso-1-cytochrome c gene of yeast; the sequence determination of RNAs and DNAs including regions of double-stranded DNA; the introduction of defined site-specific point mutations into bacteriophage OX174 and in the in vitro selection of mutant DNA from a mixture with wild-type DNA.