Transcriptions occur in mouse preimplantation embryos as early as one-cell stage. However, our understanding on gene expression at this stage is lacking. The present study applied suppression subtractive hybridization (SSH) to compared gene expression profiles of mouse zygote and oocyte. Forty-four differentially expressed genes were selected and shown positive signals by reverse dot-blot hybridization. DNA sequences comparison of these putative clones with the GenBank/EMBL databases using BLAST search identified 38 clones with >90% identity to known genes and six novel clones with less than 70% homology to the databases. Eleven out of the 44 differentially expressed clones were either originally isolated from male embryo or testis-specific genes, suggesting that these genes may be derived from paternal genome. Five differentially expressed genes of interest, including bromodomain-containing protein BP75, spindlin, radixin, pituitary tumor-transforming gene (PTTG), and proteoglycan core protein (serglycin) were further studied by semi-quantitative RT-PCR. It is noted that spindlin which involves in cell division is highly expressed in zygote, suggesting that this protein may play an important role in zygotic gene activation (ZGA) and early stage development in 1-cell stage mouse embryos. (C) 2003 Elsevier Science (USA). All rights reserved.