Truncated form of the receptor for thrombopoietin. The WSXWS motif appears to be necessary for proper protein folding and thereby efficient intracellular transport and cell-surface receptor binding. The box 1 motif is required for JAK interaction and/or activation. This protein is synthesized as an Env-Mpl polyprotein. Belongs to the type I cytokine receptor family. Type 1 subfamily.