Approximately 60% of DNA synthesis is performed for PCR use with the remaining 40% performed for general sequencing, mutagenesis studies, gene construction, special antisense constructions, and for physical studies of certain species. There is also a growing use of DNA probes in the determination of bacterial and viral contaminants, genetic abnormalities, and for the study of gene receptor sites involved in the action of various drugs. Antisense applications are substantially increasing, especially with synthesizers capable of producing modified nucleic acids. In antisense work, research oligonucleotides are designed and synthesized to compete with specific DNA and subsequently injected into infected cells. This competition alters protein production, thereby interfering with the production of bacterial toxins and viral replication.