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Transcription-based amplification methods include nucleic acid sequence-based amplification (NASBA), transcription-mediated amplification (TMA), and self-sustained sequence replication (3SR). The process is isothermal and uses the combined actions of reverse transcriptase, RNase H and RNA polymerase. The method may be applied to single-stranded RNA or double-stranded DNA targets. As in PCR, all reagents can be included in one mixture, amplification is exponential and the process can be completed in less than an hour.
Polymerase chain reaction (PCR) was the first nucleic acid amplification method developed and until now has been the method of choice since its invention by Mullis.  PCR is the preferred method for application oriented fields involving nucleic acid amplification for its simplicity, easier methodology, extensively validated standard operating procedure and availability of reagents and equipments. However, PCR has a good no of limitations, including high cost of equipment, contamination chances, sensitivity to certain classes of contaminants and inhibitors, requirement of thermal cycling etc.  These limitations gave birth to alternative methods such as loop mediated isothermal amplification (LAMP), nucleic acid sequence based amplification (NASBA), self-sustained sequence replication.
44); Human immunodeficiency virus [HIV] disease (042); Infection with drug-resistant microorganisms, unspecified, with multiple drug resistance (V09. 91) MSH: AIDS Serodiagnosis; Anti-HIV Agents; CD4 Lymphocyte Count; Counseling; Culture Techniques; Dideoxynucleosides; Drug Resistance, Multiple, Viral; Drug Resistance, Viral; Enzyme-Linked Immunosorbent Assay; Fluorescent Antibody Technique; Fluorescent Antibody Technique, Indirect; Genotype; HIV Infections; HIV-1; HIV-2; Informed Consent; Microbial Sensitivity Tests; Phenotype; Polymerase Chain Reaction; Reverse Transcriptase Polymerase Chain Reaction; Risk Reduction Behavior; Self-Sustained Sequence Replication.
The NASBA techniques like transcription mediated amplification (TMA) and self-sustained sequence replication (3SR) mimic in vivo retroviral replication mechanisms to produce RNA amplicons from an RNA template. Modified cDNA is formed from an RNA template, which is then rapidly amplified into RNA amplicons. The complete process is a single step and proceeds in a single volume with an ssRNA product suited for direct use with hybridization probes making NASBA very appealing for point-of-care Technologies (POCT) .