During early embryonic development, besides the ubiquitously expressed genes, there are also those whose expression is temporally or spatially regulated for the maintenance of normal development, growth, and differentiation. Embryos in culture have been shown to exhibit developmental delay when compared to those in vivo. This has been suggested to be caused by the delay or failure in the expression of genes required for specific stage of development. Identification of genes whose expression is required at different stages during early embryonic development might aid in the formulation of more appropriate medium for in vitro culture of embryos. These genes could also be used as markers for monitoring embryonic development. Differential display was employed to investigate the presence of stage-specific transcripts in mouse embryos during early embryogenesis. Pre-cycling female mice were superovulated with intraperitoneal injection of PMSG and hCG. They were mated and 2-cell embryos were flushed from the oviduct 42 hours post-hCG stimulation. Embryos at other stages of development were also harvested at different time. Total RNA was isolated from the embryos and was used for differential display after treatment with DNase to remove any residual DNA. An apparent 2- and 4-cell stage-specific transcript was identified and cloned by TA cloning. The cDNA insert was sequenced and was used to screen a mouse 2-cell embryo cDNA library in pBluescribe (gift from Dr. Barbara Knowles, Jackson Laboratory). Three groups of hybridizing clones were obtained with the clones carrying the longest cDNA insert in each group being D5, D11, and D17, respectively. Nucleotide sequence determination showed that the D17 cDNA was highly homologous to the chimpanzee glutathione reductase antigenic epitope sequence. D5 and D11, being 1154 bp and 1074 bp respectively, represented two novel cDNAs which did not share significant homology with any known sequence in the GenBank. Reverse transcription - polymerase chain reaction with primers specific for D5 and D11 demonstrated that D5 was expressed only in the embryo while D11 was expressed in both the embryo and adult tissues. Experiments are underway to further characterise these two cDNAs. This work was supported in part by NIH grant HD31553.