Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) thrombocytopenia-absent radius syndrome ?
    ### Response:
    Thrombocytopenia-absent radius (TAR) syndrome is characterized by the absence of a bone called the radius in each forearm. Affected individuals also have a shortage (deficiency) of blood cells involved in clotting (platelets). This platelet deficiency (thrombocytopenia) usually appears during infancy and becomes less severe over time; in some cases the platelet levels become normal.
    