(A) Diagram of the genomic region of RanBP2 disrupted by insertion trap mutagenesis with a bicistronic reporter vector between exon 1 and 2. The bicistronic transcript produces two proteins under regulation of RanBP2. Upon splicing of RanBP2, a fusion between exon 1 and β-geo (a fusion between the β-gal and neo genes) is generated, while human placental alkaline phophatase (PLAP) is independently translated using the internal ribosome entry site. Consistent with previous studies, the expression of the former is directed to cell bodies, while expression of the latter is targeted to the axonal processes [67,68]. Transcriptional 5′ RACE analysis detects a fusion between exon 1 and β-geo.