a, Putative 3’UTR stems were manipulated in the context of a Venus reporter in vivo, followed by Venus quantitation with flow cytometry. b, PRC1 3’UTR structure was mutated and compensated in Venus reporter system. For all data, reported p-values relative to wildtype Venus levels, calculated by two-sided t-test (p < .01, .001, and .0001represent *, **, and *** respectively). Venus signal was normalized to cell size with fold change reported relative to Venus levels seen with the wild type stem. All results shown are derived from four measurements: two biological and two technical replicates. Error bars show standard deviation. c, Secondary structure of functional PRC1 3’UTR stem, shown with raw DMS signal for in vivo and denatured samples. Position 1 = chrXIII:863554. d,Weakly structured 3’UTRs in vivo were tested for function as in (b) but reveal little effectwhen mutated and no evidence for compensation.