Mammalian development is orchestrated by a variety of cellular proteins with expression that is regulated precisely. Although some of the genes encoding these factors have been identified, largely by homology to those of simpler organisms, the majority of them presumably remain unknown. We report here on the results of a large-scale genetic screen that can potentially lead to the identification of many of these unidentified genes in mice. The method we developed takes advantage of the fact that many of the factors that regulate early development are expressed at highly specific stages of early embryogenesis. We therefore established a method for tagging candidate developmental genes by virtue of their expression in a stage-specific manner during formation of embryoid bodies without a bias for their expression in undifferentiated embryonic stem (ES) cells. Of 2,400 ES cell clones with random insertions of retroviral vectors carrying a human placental alkaline phosphatase reporter gene (AP), 41 clones exhibited stage-specific reporter gene expression during embryoid body formation. The majority of these insertions were in genes that are not expressed in undifferentiated ES cells. Eleven ES cell clones with characteristic patterns of AP reporter gene expression in vitro were chosen for further examination in vivo for AP expression in developing embryos. Ten ES cell clones exhibited AP expression between day 7.5 and day 10.5 of development. Clones that showed restricted reporter gene expression in vitro also exhibited similar temporally and spatially restricted AP expression in vivo. Sequence analysis of genomic DNA flanking several vector insertions and corresponding cDNAs suggested that several of the insertions identified a previously unidentified gene. Thus, screening for reporter gene expression during embryoid body formation provides an efficient means of enriching clones that contain vector insertions into potentially novel genes that are important for regulating different stages of early postimplantation development.
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