Plasma/serum TPO levels are decreased in GPIbα-deficient mice and human BSS patients. (A) ELISA of serum TPO levels (1:10 dilution) in WT, GPIbα^−/−, and β3^−/− mice (all on BALB/c background; n = 6-10). ELISA of plasma TPO levels (1:2 dilution) in WT, GPIbα^−/−, and heterozygous (GPIbα^+/−) mice on BALB/c background (n = 6-10) (B) and WT and VWF^−/− mice on C57BL/6J background (n = 3) (C). (D) ELISA-determined plasma TPO concentration in BSS patients (n = 7) and healthy donors (n = 99). **P < .01, ****P < .0001. NS, not significant.