Dear Editor,rnDNA N6-methyladenine(6mA),one of the most prevalent epigenetic base modifications in prokaryotes,1 is recently found in multicellular eukaryotes.2-8 This nucleobase may have epigenetic roles in regulation of retrotransposons,chromatin organization,and so on.2-8 However,both our group9 and Greer's group10 noticed that eukaryotic DNA is easily contaminated with a minute of bacterial DNA,which carries overwhelmingly abundant 6mA(～2％6mA/dA).1 This brings great challenges for accurate detection of DNA 6mA in eukaryotes in terms of both sample pretreatments and analytical technologies.9,10 For example,inconsistent with the report of Wu et al.,8 Schiffers et al.failed to detect 6mA above background levels in mouse embryonic stem(mES)cells using sensitive ultra-high-performance liquid chromatography-quadruple mass spectrometry(UHPLC-MS/MS)analysis.11 To date,it is of intensive interest to seek conclusive evidence to support the prevalence of this post-replicative adenine modification in mammals.