SB transposon insertions in Arnt and Gdi2 . A, For cancer gene identification, the SB transposon is designed to mimic retroviral insertional mutagenesis. The transposon contains sequences from the 5’ long terminal repeat (LTR) of murine stem cell virus (MSCV) which serve as promoter/enhancer elements to activate transcription of genes into which the transposon has inserted. The MSCV LTR is followed by a splice donor (SD). Thus, a transcript initiated in the LTR (mRNA1) can splice into downstream exons of the target gene. The transposon also contains splice acceptor (SA) sites and polyadenylation (pA) sequences to truncate transcription of target genes (mRNA2). The SB transposon schematized here is the T2/Onc vector . B, Insertion maps showing the sites of transposon insertion (arrowheads) in mouse genes Arnt and Gdi2. Arrow heads point in the direction of transcription from the MSCV LTR. Transcription-activating insertions in the 5’ untranslated region and intron 1 are indicated by solid arrowheads. Coding exons are represented by vertical lines, introns by the interval spaces. Right-angle arrows indicate the direction of gene transcription.