(A) Targeted inactivation of the C/EBPɛ gene. Restriction map (Top) of the murine locus containing the C/EBPɛ gene. The 6.9-kb EcoRI 5′ fragment and the 4.0-kb HindIII 3′ fragment used for targeting vector construction are shown (Middle). (B) Southern blot analysis of BamHI-digested genomic DNA extracted from a litter of a heterozygote cross. The 13.6-kb wild-type (WT) fragment from the nontargeted allele and the 8.1-kb targeted (KO) fragment from the mutated allele are shown. The genotypes are indicated above the lanes, +/+ represents wild-type, +/− heterozygous, and −/− mutant animals. The blot was probed with the HindIII–EcoRI flanking probe indicated in A. (C) Northern blot analysis of RNA extracted from bone marrow. The presence of the C/EBPɛ transcripts in +/+ and +/− animals is shown. The blot was probed with part of exon 1 of C/EBPɛ and reprobed with β-actin. (D) Early lethality in C/EBPɛ −/− mice. Mortality of C/EBPɛ −/−, 129/SvEv pure background (□), 129Sv/NIH Black-Swiss mixed background (⋄), C/EBPɛ +/−, and +/+ (○) are shown.