A gene that makes a protein called the thrombopoietin receptor. This is a protein that helps control the number of blood cells that are made in the bone marrow, especially platelets. Mutated (changed) forms of the MPL gene may cause the body to make abnormal blood cells or too many platelets. Mutations in this gene have been found in some types of blood conditions, including essential thrombocythemia and primary myelofibrosis.