Figure 1.

Strategy for generating sCSF1 K/O mice. (a) Splicing pattern for the CSF1 gene. (b) The targeting construct for deletion of sCSF1 introduces a stop codon followed by a lox-p site into exon 6 at a position 3′ to the splicing site for sCSF1. The stop codon is designed to include all three reading frames. The Neo cassette with flanking frt sites (lox-p-frt-Neo-frt) is introduced into intron 5. Excision of the Neo cassette with flp recombinase leads to a CSF1 allele that contains two lox-p sites, one in intron 5, and one which is downstream of the splice site for sCSF1. In animals bearing two of these floxed alleles (that is, before cre-mediated recombination) both isoforms of CSF1 will be generated. Splicing to the sCSF1 splice site will lead to generation of an mRNA that includes the sequences for the proteolyitc cleavage sites for sCSF1, followed by a stop codon. The protein product of this mRNA will yield normal mature sCSF1 since it contains the appropriate signals for proteolytic cleavage with the surrounding sequences. Recombination with cre leads to selective deletion of the splice acceptor site for the sCSF1 isoform. This leaves the splice acceptor site for mCSF1 intact. Thus, a mature functional mCSF1 will be generated, but no sCSF1.