It turns out the mouse endogenous retrovirus L (MERVL) is essential for embryogenesis, according to a recently published article in Nature Genetics. From the Discussion (emphasis added):

We provided functional evidence that transcriptional activation of MERVL is essential for progression of development in mouse preimplantation embryos. Depletion of MERVL transcripts results in embryonic lethality with profound defects in development and is associated with dysregulation of MERVL including their adjacent transcripts, and retaining two-cell-like transcriptome and chromatin state (Fig. 6i). These findings suggest the possibility that MERVL transcription in totipotent cells may act as a switch for the transition from totipotency to pluripotency and is responsible for the onset of differentiation and ontogeny.

For more, see “Transcription of MERVL retrotransposons is required for preimplantation embryo development” (open access).