In the case of bacteria, transposition mutagenesis is usually accomplished by way of a plasmid from which a transposon is extracted and inserted into the host chromosome. This usually requires a set of enzymes including transposase to be translated. The transposase can be expressed either on a separate plasmid, or on the plasmid containing the gene to be integrated. Alternatively, an injection of transposase mRNA into the host cell can induce translation and expression. Early transposon mutagenesis experiments relied on bacteriophages and conjugative bacterial plasmids for the insertion of sequences. These were very non-specific, and made it difficult to incorporate specific genes. A newer technique called shuttle mutagenesis uses specific cloned genes from the host species to incorporate genetic elements. Another effective approach is to deliver transposons through viral capsids. This facilitates integration into the chromosome and long-term transgene expression.