A plasmid DNA vector is described that is suitable for cloning synthetic DNA sequences. These cloned synthetic DNA sequences can be transcribed in vitro to produce oligoribonucleotides of defined sequence. Transcription is directed by a promoter based on the consensus sequence for Escherichia coli promoters and uses E. coli RNA polymerase. The vector is useful for cloning oligodeoxyribonucleotides of mixed sequences, the individual sequences being resolved by transformation and colony selection. Oligoribonucleotide synthesis from the vector is highly specific. Application of these sequences in hybridization experiments is demonstrated.