ABSTRACT
This chapter describes that defective interfering (DI) particles derived from RNA viruses and, based on the convention for RNA viruses, is subdivided into DI particles obtained from positive and negative strand RNA genomes. RNA isolated from positive strand RNA viruses is infectious. DI RNAs can also express their biological activity when transfected into cells in the presence of helper virus. The generation of defective molecules requires only that aberrant replicative events occur. The ability to identify what properties of defective RNAs are required for amplification requires that the sequence of defective genomes and that of the nondefective genome from which they are derived be determined. The picornaviruses have been distinctive among RNA viruses with respect to both the number of passages required to detect defective particles and the relatively small size of the deletions. The deletions in the DI RNAs have all been mapped to a narrow region between 1300 and 3100 nucleotides from the 5' terminus.
