Transcription-based ITA methods have been developed that generate amplified RNA from RNA templates (Deiman et al., 2002), although an initial denaturation step must be included when using dsDNA targets (Guatelli et al., 1990). The RNA amplicons produced in the reactions serve as targets for the production of additional dsDNA templates, and this cycling gives exponential amplification kinetics to the reaction. The basic concept for the exponential amplification of a target nucleic acid sequence is modeled on the strategy of retroviral replication (Guatelli et al., 1990). Transcription-based assays have been developed using different names including NASBA (nucleic acid sequence-based amplification) (Deiman et al., 2002),
TMA (transcription-mediated amplification), TAS (transcription-based amplification system) (Kwoh et al., 1989), and 3SR (self-sustained sequence replication) (Fahy et al., 1991, Guilfoyle et al., 1997). All these methods use the same basic concept for exponential amplification of a target nucleic acid sequence and are modeled on the strategy of retroviral replication. For purposes of simplifying the terminology, we will refer to the general method as a 3SR reaction in the following text.
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