Cebpa cDNA and a neomycin resistance gene in opposite orientation replaced exon 1 of the alpha fetoprotein locus. Quantitative RT-PCR of fetal livers demonstrated that mutants had a higher level of Cebpa mRNA than wild-type livers. It was undetermined whether those higher levels were due to expression of the knock-in gene alone or due to a combination of the knock-in gene plus elevated expression of the endogenous gene. (J:103404)