The H-2 complex of mice contains many genes in addition to the gene families involved in immune reactions. Some of them are believed to function in mouse development, as suggested by the findings that several embryonic lethal mutations map within or near the H-2 complex. We have analyzed the H-2K/tw5 region in an attempt to study non-H-2 genes encoded in this region. Overlapping cosmid clones spanning about 170 kilobase pairs of DNA, including the H-2K/tw5 region of the mouse, have been screened for genes expressed in embryonic carcinoma cells. A transcript of 2.8 kilobase pairs (K. Abe. J.-F. Wei, F.-S. Wei, Y.-C. Hsu, H. Uehara, K. Artzt, and D. Bennett, EMBO J. 7:3441-3449, 1988) encoded by the KE 4 gene flanking H-2K distally was identified. The transcript was abundantly expressed in embryonic carcinoma cells but was present at low levels in other tissues in adults. A cDNA for this transcript was isolated from the F9 embryonic carcinoma cell line and sequenced. It potentially encodes a protein of 436 amino acids with several interesting features. First, it contains two regions made of well-conserved repeats unusually rich in histidine residues. In the repeats, histidine alternates with other amino acids, notably glycine or serine. Second, the two histidine-rich regions are separated by three putative membrane-spanning domains. Third, the N-terminal part of the sequence shows characteristics of a signal peptide. The results indicate that the protein coded by the gene may be a transmembrane protein with histidine-rich charge clusters. A similar sequence motif found in other known genes allows speculation on the possible functional of this gene.