This modification involves the addition of a methyl group (-CH3) group to the 6th nitrogen on the adenine base in an mRNA molecule. This was among the first mRNA modifications to be discovered in 1974.  This modification is common in viral mRNA transcripts and is found in nearly 25% of them. The distribution of the modification not uniform with some transcripts containing more than 10.  mA modifications are a dynamic process with many applications ranging from viral interactions with cellular machinery and structural adjustments to viral life cycle control. Studies have shown  different regulatory patterns for different viruses depending on the context. For single stranded RNA viruses, the effects of the modifications appear to differ on the basis of the viral family. In the HIV-1 genome, the single stranded positive sense RNA contains mA modifications at multiple sites in both the untranslated and coding regions. The presence of this modifications in the viral transcript is enough to increase corresponding modifications in host cell mRNA through binding interactions between the HIV-1 gp 120 envelope protein, and the CD4 receptor in T lymphocytes without causing a corresponding increase in.  For HIV-1 and other RNA viral families like chikungunya, enteroviruses and influenza, studies show both a positive and negative role for mA modifications on viral life replication and infection. For other families, the role effects are clearer. For the flaviridae family, the modification had a negative role and hindered viral replication.  The modification in respiratory syncytial virus families showed a positive role and enhanced viral replication and infection. The causes of these apparently different roles from different responses within the same family of viruses and why the viral families like flaviridae conserve mA modifications when they negatively impact their cycles are currently unknown and under investigation.
Most of the RNA viruses carry out their cycles in the cytoplasm, away from the required machinery for writing and erasing mA modifications which are housed in the nucleus. For DNA viruses, that cycle in the nucleus with direct access to said machinery, no clear general positive or negative regulatory role can be attributed to mA modifications. In the simian virus and hepatitis B viruses, different mA reading complexes were shown to have different roles  in regulation with some having a conserved positive role and others having a neutral or negative effect on replication.