On Oct. 24, 2024, researchers from Children’s Medical Center Research Institute at UT Southwestern reported that ancient viral remnants in the human genome are activated during pregnancy and after significant bleeding in order to increase blood cell production. This was an important step toward defining the purpose of “junk DNA” in humans.

Retrotransposons are ancient viral gene sequences now permanently part of our genome and sometimes called “junk DNA” because they don’t encode proteins that contribute to cellular function. They use an enzyme called reverse transcriptase, just like the human immunodeficiency virus (HIV), to replicate themselves.

Humans have evolved mechanisms to keep retrotransposons turned off, most of the time, because retrotransposons have the ability to damage DNA when they replicate and reinsert into other parts of the genome.

The scientists performed analyses on blood samples from pregnant women, including three people being treated with reverse transcriptase inhibitors, and found retrotransposons and interferon are also activated in human blood-forming stem cells during pregnancy, in addition to being necessary to avoid anemia. The study was published in Science.