Romiplostim-induced increase in platelet number and size. (A) Platelet counts of WT and Myh9^−/− mice before (day 0) and after 8, 18, and 28 days of romiplostim administration. Each bar represents the mean ± SEM of 5 animals. (B) Relative platelet size variation during romiplostim treatment as visualized by flow cytometry (forward scatter parameter) in the GPIbβ-positive population. Mean ± SEM of 10 animals. Of note, the platelet size increase after start of romiplostim was transient in the Myh9^−/− mice, indicating that it is not only the effect of TPO receptor stimulation.