RT-PCR analysis of the expression of the tk gene driven by the α_IIb promoter. A 5-μL aliquot of the cDNA derived from BM cells was amplified with a pair of specific primers for 30 cycles. One fifth of the product was applied to 1.5% agarose gel for electrophoresis. Panels show the results of Southern hybridization of RT-PCR products with the internal probe of each gene. Each experiment was checked for reproducibility. Lane 1, nontransgenic mouse; lane 2, HαIIbtk (1 copy); lane 3, MαIIbtk (1 copy); lane 4, MαIIbtk (3 copies); lane 5, MαIIbtk (4 copies); lane 6, PCR performed without RT as control against contamination. Amplification of endogenous α_IIb transcripts were performed with α_IIb-specific oligonucleotides on the same samples.