FlnA and GPIbα regulate platelet size. (A) Increased surface expression of GPIbα is a marker of megakaryocyte maturation and differentiation, whereas FlnA expression levels increase as early as in ES cells. Ablation of FlnA expression in FLNA^loxP PF4-Cre megakaryocytes results in the premature production of enlarged platelets with reduced GPIbα expression.⁴  Kanaji et al show that FlnA and FlnB regulate ES cell maturation and differentiation.¹  (B) Balanced expression levels of FlnA and GPIbα in megakaryocytes regulate the production of normal-sized platelets. FlnA-null and GPIbα-null platelets, as well as platelets over-expressing GPIbα, are enlarged. Conversely, FlnA subcellular localization is altered in GPIbα-null platelets and in HEK293 cells overexpressing GPIbα.