The transcription factor LFB1/HNF1 from rat liver nuclei is a 628 amino acid protein that functions as a dimer binding to the inverted palindrome GTTAATN-ATTAAC consensus site. We have crystallized a 99 residue protein containing the homeodomain portion of LFB1, and solved its structure using X-ray diffraction data to 2.8 A resolution. The topology and orientation of the helices is essentially the same as that found in the engrailed, MAT alpha 2 and Antennapedia homeodomains, even though the LFB1 homeodomain contains 21 more residues. The 21 residue insertion is found in an extension of helix 2 and consequent lengthening of the connecting loop between helix 2 and helix 3. Comparison with the engrailed homeodomain-DNA complex indicates that the mode of interaction with DNA is similar in both proteins, with a number of conserved contacts in the major groove. The extra 21 residues of the LFB1 homeodomain are not involved in DNA binding. Binding of the LFB1 dimer to a B-DNA palindromic consensus sequence requires either a conformational change of the DNA (presumably bending), or a rearrangement of the subunits relative to the DNA.