Target validation for EXT1. (A) shRNA vectors targeting different regions of the EXT1 transcript were tested for their ability to induce an effect on erythroid progenitor activity as measured by frequency of erythroid progenitors 1 week after transduction Results are given as mean values (± SEM); *P < .05 (Student t test). (B) The same constructs were tested for their ability to knock down the EXT1 transcript as measured by quantitative (q)–PCR. Results from 3 independent experiments are shown. To assess differences in relative knockdown efficiency, a paired t test was used to test significance when comparing shEXT1 and shEXT1-3 with any of the other shRNAs. P values are indicated for the comparisons with shEXT1-4 and shEXT1-5. For comparisons with the other shRNAs P values were below .02.