During replacement gene therapy, we normally think of replacing the whole defective gene with a complete functional copy. However, some genetic defects consist of just a single base change, or perhaps a cluster of closely linked base alterations. In this case, the defective gene could be “patched” rather than replaced. This may be done by crossing over with a relatively short double-stranded oligonucleotide, which carries the wild-type sequence.
The crossover frequency may be improved by using an RNA-DNA hybrid oligonucleotide. Such hybrids are known to take part in crossover formation more readily than double-stranded DNA. In addition, designing hairpin bends protects the ends of the oligonucleotide and prevents degradation by exonucleases. This technique is still in the experimental phase.