There has been little systematic investigation of the phenomenon of this type of mutational asymmetry in RNA viruses, although it has been long recognised that an RNA editing enzyme, ADAR1 may play a role in prominent excess of U->C and A->C mutations in measles virus genomes associated with sub-acute sclerosing panencephalitis (SSPE) [28], while APOBECs have been shown to create hypermutated proviral DNA copies of HIV-1 and other retroviral genomes [23,29,30]. Here, ADAR is linked to subacute sclerosing panencephalitis.