Exons 3 and 4, encoding the conserved helix-hairpin-helix domain, were deleted by the insertion of a neomycin selection cassette. The absence of full-length transcript was verifed via RT-PCR analysis. The expression of the adjacent Tsc2 was unaffected. Sequence analysis revealed that should an aberrantly spliced transcript arise via readthrough of the neo transgene, the splicing of exon 2 to exon 5 would generate a frameshift mutation that precludes production of a functional protein.