Inhibition of WT-PF4 with the nontetramerizing mutant PF4^K50E. Inhibition of PF4 tetramerization (A), ULC formation (B), and FcγRIIA activation (C). PF4 concentrations were maintained constant by diluting WT PF4 with PF4^K50E. Data are the mean ± SEM of at least 3 independent experiments performed in triplicate.