SCF transgene constructs. Transgene expression of the mSCF²⁴⁸ and mSCF^X9/D3 cDNAs in founder mice was achieved using a minigene cassette consisting of mSCF²⁴⁸ or mSCF^X9/D3 cDNA expressed from the human PGK promoter. The PGK promoter, consisting of 514 bp 5′-flanking sequence from the X-linked human phosphoglycerate kinase-1 gene,54 up to but excluding the translational start codon, was subcloned into pGEM-7 as an Aatll/Sph l fragment. The mSCF²⁴⁸ or mSCF^X9/D3 cDNA in addition to a splice donor/acceptor and poly A site was subcloned from the expression plasmid V19.8mSCF²⁴⁸ or V19.8mSCF^{X9/D3 36} as aXho l/Kpn l fragment 3′ to the PGK promoter in the pGEM-7 plasmid vector to generate the final transgene plasmid. hPGKpr: human phosphoglycerate kinase promoter: SD/SA: splice donor/splice acceptor sequences; mSCF²⁴⁸ cDNA: murine SCF cDNA encoding full-length soluble SCF protein: mSCF^X9/D3 cDNA: murine SCF cDNA encoding full-length MR¹⁹ SCF protein; poly A: polyadenylation sequence. (X) denotes the location of an insertedXho l at the proteolytic cleavage site in exon 6, and (▿) denotes a 12-bp deletion of the secondary proteolytic cleavage site in exon 7.