a, Outline of the experiment used to simulate a mixed population of T cells with intact UMPS or bi-allelic UMPS knockout. Pure cell populations with the respective genotypes were stained with different tracking dyes (eFluor670 or CFSE), mixed and cultured in the presence or absence of 5-FOA during continuous T cell stimulation. b, Representative FACS plots (out of the 3 experiments) showing the mixed T cell populations before and 3 days after culture with 5-FOA. The relative percentages of the populations and the genotype of the cell populations identified by the dyes are annotated. c, Representative plots used for gating of viable T cells (plots 1 and 2) upstream of identification using the tracking dyes (plot 3) are shown. PI, propidium iodide. KO, knockout. WT, wild-type.