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The Australian Stem Cell Centre (ASCC), developed the following fact sheets in respect of stem cell science and related topics. Please note that whilst the Fact Sheets were updated shortly before the ASCC closed in 2011, users should note that they are no longer being updated and be aware that the content in them may be out of date. Although the content of the fact sheets is unlikely to have been superseded please do not rely on them for any decision-making purposes and they should be used as part of a wider reading program on the subjects.

The most recent human retroviral infections leading to germ line integration took place with a subgroup of human endogenous retroviruses called HERVK(HML-2). The human genome contains ~90 copies of these viral genomes, which might have infected human ancestors as recently as 200,000 years ago. HERVs do not produce infectious virus: not only is the viral genome silenced - no mRNAs are produced - but they are littered with lethal mutations that have accumulated over time. A recent study revealed that HERVK mRNAs are produced during normal human embryogenesis. Viral RNAs were detected beginning at the 8-cell stage, through epiblast cells in preimplantation embryos, until formation of embryonic stem cells (illustrated). At this point the production of HERVK mRNA ceases. Viral capsid protein was detected in blastocysts, and electron microscopy revealed the presence of virus-like particles similar to those found in reconstructed HERVK particles. These results indicate that retroviral proteins and particles are present during human development, up until implantation.