Abstract: This invention provides a method for producing a mutant virus or viroid, or a mutant recombinant RNA molecule comprising a single stranded virus genome or a single stranded viroid genome, a circular virus genome or a circular viroid genome, or a segment of a virus genome or a segment of a viroid genome which comprises incubating a recombinant single-stranded RNA molecule comprising a recognition sequence for the binding of an RNA directed RNA polymerase, a sequence for the initiation of product strand synthesis by the polymerase and a heterologous sequence of interest derived from a different RNA molecule inserted at a specific site in the internal region of the recombinant molecule under appropriate selective conditions and for a sufficient period of time permitting the selection of a mutant population, the heterologous sequence of interest comprising the RNA of the single stranded virus genome or the single stranded viroid genome, the circular virus genome or the circular viroid genome or a segment of the v