Fig. 8. Identification of m⁵U in mRNA of HEK293T cells using stable isotope tracing monitored by mass spectrometry. (A) DMEM medium supplied with D₃-Met was used for the cell culturing. D₃-Met could be converted to D₃-SAM that is a methylating reagent for methylation of nucleic acids. (B) The extracted-ion chromatograms of the CMCT labelled m⁵U standard (upper panel), detected CMCT labelled m⁵U from mRNA of HEK293T cells (middle panel), and detected CMCT labelled D₃-m⁵U from mRNA of HEK293T cells (bottom panel).