Little is known about the molecular control of mammalian preimplantation development. It is proposed that a cDNA library approach be used to study the expression of genes during the preimplantation period of embryogenesis. Existing cDNA libraries will be screened with probes for genes known to be abundantly expressed during this stage of development to establish the quantitative nature of the libraries. Different approaches will then be used to extract information from these libraries: screening of gridded cDNA arrays, PCR-screening of the libraries and colony hybridization will be done using degenerate oligonucleotides containing DNA-binding motifs of transcription factors. Expression of possible embryo-unique genes thus identified will then be traced through developmental time using RT-PCR, northern blot analysis and in situ hybridization. Finally, chromosomal mapping of these genes will be done and all the information incorporated into an on-line database designed so that all the relevant information is easily accessible.