the large amount of endogenous long rna is an important open question in cell biology. several reports suggest that phosphorylation of the 5'-end of a long rna molecule can influence its immunogenicity, and specifically that 5'-triphosphate rna, which can be produced during viral infection, is more immunogenic than 5'-diphosphate rna, 5'-monophosphate rna or rna containing no 5' phosphate. however, in vitro-transcribed (ivt) long rna containing a 7-methylguanosine cap (present in eukaryotic mrna) is also highly immunogenic despite having no 5' phosphate, suggesting that characteristics other than 5'-phosphorylation can influence the immunogenicity of an rna molecule. eukaryotic mrna contains chemically modified nucleotides such as n6-methyladenosine, 5-methylcytidine, and 2'-o-methylated nucleotides. although only a very small number of these modified nucleotides are present in a typical mrna molecule, they may help prevent mrna from activating the innate immune system by disrupting secondary structure that would resemble double-stranded rna (dsrna), a type of rna thought to be present in cells only during viral infection. the immunogenicity of long rna has been used to study both innate and adaptive immunity. repeated long-rna transfection inhibiting only three proteins, interferon-β, stat2, and eif2ak2 is sufficient to rescue human fibroblasts from the cell death caused by frequent transfection with long, protein-encoding rna. inhibiting interferon signaling disrupts the positive-feedback loop that normally hypersensitizes cells exposed to exogenous long rna. researchers have recently used this technique to express reprogramming proteins in primary human fibroblasts.