THPO and congenital amegakaryocytic thrombocytopenia 1: MPL or THPO knockout mice appear hematologically normal except for thrombocytopenia but have decreased marrow HSC (24).In contrast, in human congenital amegakaryocytic thrombocytopenia (CAMT), MPL mutations, usually in the MPL distal extracellular cytokine receptor homology domain (CRHD) (25), cause thrombocytopenia, elevated plasma THPO, and severe marrow aplasia (26).