The Ped (preimplantation embryonic development) gene influences the rate of mouse preimplantation embryonic development and subsequent survival. Four similar tandem genes in the Q region of the major histocompatibility complex-Q6, Q7, Q8, and Q9-were identified as Ped gene candidates. In this study, expression of these genes during preimplantation development was examined and quantitated by reverse transcription-polymerase chain reaction and single nucleotide primer extension assays in order to investigate their contribution to the Ped gene phenotype. The Q7/Q9 gene pair was found to be transcribed in preimplantation mouse embryos, whereas transcription of the Q6/Q8 gene pair was undetectable. Both Q7 and Q9 are expressed in embryos from one Ped fast strain, C57BL/6, while only the Q9 gene is expressed in another Ped fast strain, B6.K2. These results suggest that both the Q7 and Q9 genes can function as the Ped gene in the mouse. Interestingly, the expression pattern of the Q7 and Q9 genes in preimplantation embryos is the same as in splenic lymphocytes. However, the Q6 and Q8 genes are expressed in splenic lymphocytes but not in preimplantation embryos. Treatment of mouse preimplantation embryos with interferon gamma (gamma-IFN) did not induce expression of the Q6/Q8 genes but enhanced expression of the Q7/Q9 genes. The mechanism of this differential transcription pattern is currently under investigation.