We have marked a cloned mouse IAP sequence with a neomycin-containing indicator gene whose expression is conditioned by passage of the transposon through an RNA intermediate. Transposition of the marked IAP introduced into tumor cells could be detected by simple selection of the cells in G418, at a frequency of 10(-6) per cell per generation. Southern blot analysis and nucleotide sequencing after PCR amplification demonstrated "retrotransposition" of the marked element, with splicing out of an intron contained in the indicator gene, and retroviral-like reverse transcription and integration of the transposed IAPs, with 6 bp duplications of the identified target sites. Transposition was found to be mutagenic for the element, as might be expected if the identified marked and endogenous IAP transcripts were coencapsidated into IAP particles as dimers.