Extended Data Figure 6: One-carbon units used in purine and thymidine synthesis are derived from serine.

a, Serine and ATP labelling pattern after 24 h incubation of HEK293T cells with [U-¹³C]serine. The presence of M+1 to M+4 ATP indicates that serine contributes carbon to purines both through glycine and through one-carbon units derived from serine C3. b, Quantitative analysis of cytosolic one-carbon unit labelling from measured the intracellular ATP, glycine, and serine labelling reveals that most cytosolic 10-formyl-THF assimilated into purines comes from serine. c, [U-¹³C]serine labels the methyl group that distinguishes dTTP from UTP. d, [U-¹³C]glycine does not label dTTP. e, The extent of dTTP labelling mirrors the extent of intracellular serine labelling. f, Methionine does not label from [U-¹³C]glycine. In all experiments, cells were grown in [U-¹³C]serine or glycine for 48 h. Mean ± s.d., n = 3.