PF4 stimulation of aPC generation in-solution by IIa/TM complex. aPC generation was monitored using the chromogenic substrate S2366. Reaction mixtures contained IIa at either 0.2nM (A) or 2nM (B). Increasing concentrations of PF4 (WT) or PF4^K50E mutant that can dimerize but fail to form tetramers were used in the presence of TM that either contains (TM) or lacks CS side chain (TM_-CS). Each curve represents the mean ± SD of 4 or 5 experiments, each performed in duplicate. *P < .01 vs no added PF4.