HNF-36, a transcription factor of the winged-helix family, is expressed in embryonic and adult endoderm and also in midline cells of the node, notochord, and floor plate in mouse embryos. To define the function of HNF-39, a targeted mutation in the HEIF-3p locus was generated by homologous recombination in embryonic stem cells. Mice lacking HNF-3/? die by embryonic day (E) 10-11. Mutant embryos examined from E6. 5 to E9. 5 do not form a distinct node and lack a notochord. In addition, mutant embryos show marked defects in the organization of somites and neural tube that may result from the absence of the notochord. The neural tube of mutant embryos exhibits overt anteroposterior polarity but lacks a floor plate and motor neurons. Endodermal cells are present but fail to form a gut tube in mutant embryos. These studies indicate that HNF-36 has an essential role in the development of axial mesoderm in mouse embryos.

The isolation and analysis of DNA-binding proteins required for cell type-specific gene expression in vertebrates has provided insight into the molecular mechanisms that control the differentiation of specific cell types and organs (Walker et al., 1983; Ctt et al., 1984; Ciliberto et al., 1985; Costaet al., 1986; Couttois et al., 1987). Studies on the regulation of liver-specific gene expression have identified three structurally related proteins, the hepatocyte nuclear factors 3a, 38, and 3~(HNF-3a, HNF-38, and HNF-3y) that function as sequence-specific DNA-binding proteins (Lai et al., 1990, 199l). The HNF-3 genes encode a similar DNA-binding domain that is related to that of fork head (Weigel and Jackie, 1990; Lai et al., 1991), a gene required for terminal segment differentiation including gut invagination in Drosophila (Weigel et al., 1989). The conservation in sequenceof this DNA-binding domain, termed the winged-helix domain on the basis of its three-dimen-