Abstract: Double stranded DNA (ds-DNA) can be prepared by preparing a hybrid DNA containing a single stranded portion and a double stranded portion and carrying out in vivo gap repair on the hybrid DNA. The hybrid DNA is prepared by synthesizing a single strand of DNA and introducing the single strand into double stranded DNA. This method of synthesis can be used to make synthetic genes, including synthetic DNA coding for the TAT protein of HIV-I, and incorporates useful restriction sites. Also included are flanking restriction sites to simplify the incorporation of the gene into any desired expression system.