The FLI1 c.960C>T mutation in the ETS domain alters transcriptional activity of the protein. (A) Sequence traces showing the homozygous 970C>T mutation in patients II.1 and II.2 (left) and parents (right). (B) Amino acid homology in the ETS domain of FLI1 and ETS1. Arg324 is highly conserved among species. (C) HEK263 cells were transfected with an empty vector control, wild-type FLI1, or the mutant FLI1 c.970C>T transcript, and transcriptional activity was measured by the ratio of signal from the GP9, GP6, or ITGA2B promoter (firefly) compared with a control promoter (renilla). The data are expressed in fold change in luciferase activity and represent the mean ± standard error of the mean (N = 9) (*P < .05, **P < .01). (D) Western blotting of platelet lysates from affected (II.1, II.2) unaffected (I.1, I.2) family members, and unrelated controls representing 100%. Quantitation was performed using ImageLab software (*P < .05, **P < .01). (E) Ribbon representation of the structure of the FLI1 ETS domain in pink demonstrating the position of Arg324. Superimposed in green is the structure of the ETS1 ETS domain bound to DNA (gray). The N-terminal autoinhibitory region is shown in dark green and the core ETS domain is shown in light green.