May have no phenotypic effect if it is in a non-translated region. Frame shift mutation - usually deleterious, because it changes all downstream amino acids. Sometimes stop or start codons are created by insertion or deletion, resulting in an incomplete and nonfunctional gene. The only genes present make tranposases. Include transposases and other functional genes as well.
Plasmids are an example.
Common in plants, fungi, animals. Drosophila may have as many as 50 or 100. Alteration of host gene expression insertion into coding region can abolish gene function insertion into control region affects expression of the gene. Depends on the method used to detect mutation using phenotypic expression will underestimate mutation rates. Frequency is much lower.