MPL and Thrombocytosis: To test whether cytokine receptors are involved in the pathogenesis of thrombocytosis caused by mutant CALR in zebrafish, mpl, epor and csf3r MOs (each with 1 ng) were injected in 1–2 cells stage embryos of cd41:GFP line and assayed for their effects on the number of CD41+ thrombocytes at 5 d.p.f. Co-injection of CALR-del52 mutant mRNA (100 pg) with each MO was also performed in a subset of embryos.