To be clear, this is in a tumergenic immortalized cell line where you'd expect to find LINE1 upregulated compared to say, muscle tissue and dendritic immune cel

To be clear, this is in a tumergenic immortalized cell line where you'd expect to find LINE1 upregulated compared to say, muscle tissue and dendritic immune cells. Their late paper qualification that most cells don't, but hey, some neurons do, doesn't really support this as a model of what happens in human bodies in most cells most of the time. That said, it's not impossible, and certainly it could happen to viral infection created mRNA just as easily as synthetic mRNA. In fact, mammals wouldn't exist without the membrane fusion protein (to make the placenta) our long ago animal anscestors to mammals received from a retrovirus. The LINE1 elements are from a retrovirus itself.