Fig. 2. Gene expression during normal preimplantation development. Embryos of indicated developmental stages were harvested from pregnant females. Gene expression was assayed by RT-PCR in individual embryos. Gapd served as a ubiquitously expressed control. Expression of all test genes is readily detectable from the four-cell stage with the exception of Dppa1. Dppa1 expression is upregulated at the morula-to-blastocyst transition. Control reactions omitting cDNA or reverse transcriptase showed no amplification products (data not shown).