Six new genes have been found from H-2K region of mouse major histocompatibility complex (MHC). Their chromosomal location has been also known for embryonic lethal mutant, tcl-tw5. In order to identify the most likely candidate for the tw5 gene, these genes have been studied for specific expression in embryo, by in situ hybridization. Four genes (KE1, 2, 4 and Tctex-7) of those six were transcribed in early embryonic stages. KE2and Tctex-7 genes showed stronger expressions in embryonic cells than in non-embryonic cells, but signals in embryos showed a broad spatial and temporal distribution. KE1 and 4 genes were expressed in every cell, not only embryonic but non-embryonic cell. Thus, in situ hybridization data could not relate any of those genes with tw5 mutation. These results could be explained in one of the following ways: a) none of these genes represents the tw5 gene; b) the RNA of the tw5 gene may not be localized; c) expressions were lower than detectable level due to technical limitation.