Patient thrombocytosis paradoxically associated with a loss of function receptor. (A) Family trees and biological characteristics: in 2 families, probands are homozygous for the MPL P106L mutation and exhibit thrombocytosis with high levels of THPO and bone marrow megakaryocytic proliferation. (B) Platelet signaling study: STAT3 and AKT activation over a time course were examined by western blot after starvation and subsequent stimulation with THPO at 10 ng/mL or 100 ng/mL and compared with a healthy control. (C) Megakaryocytic proliferation: the MK progenitor compartment response to THPO was studied by plasma clot culture in relative percentage compared with the control or in absolute values. MK colonies were split in 2-5, 5-10, and >10 MKs per colony. The error bars represent mean ± standard deviation of triplicates from a representative experiment.