EμV_HP-TEL/PDGFβR transgenic construct. The immunoglobulin heavy-chain enhancer/promoter cassette contains the 682-bp EcoRI-Xba I fragment of the immunoglobulin-mu enhancer (Eμ), ligated to the 330-bp HindII-Eco I promoter fragment of the V_H gene (V_HP). The 3′ end of the construct consists of the 1.6-kbBamHI-Pst I fragment of the genomic human β-globin gene containing a small (19-bp) piece of exon 2, the entire 849-bp intron 2, including splice donor and acceptor sites, all of exon 3, and a fragment of nearly 500 bp of 3′UTR, including the polyadenylation signal. Into the multicloning site of pBSVE6βK, between these 2 cassettes, the 2.2-kb cDNA for TEL/PDGFβR fusion gene was inserted. The stop codon in TEL/PDGFβR prevents the β-globin exons from being translated, but the existence of these exons in the transgene mRNA was exploited to assay for expression. Primers used in RT-PCR are indicated by small arrows. Restriction sites: B2 =BssHII, B1 = BamHI, R1 = EcoRI, S2 =SacII, H = HindIII.