MPL and neoplasm: To investigate whether GPIbα deficiency‐attenuated tumor metastasis is related to decreased platelet activation, basal levels and tumor cell‐induced surface exposure of P‐selectin and phosphatidylserine (PS), which are two markers for platelet activation and have been shown to mediate the tumor cell–platelet interaction [37, 38, 39], were compared among WT, Gp1ba−/−, and Mpl−/− platelets.