Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Sickle Cell Disease ?
    ### Response:
    Abnormal hemoglobin, called hemoglobin S, causes sickle cell disease (SCD).
                
The problem in hemoglobin S is caused by a small defect in the gene that directs the production of the beta globin part of hemoglobin. This small defect in the beta globin gene causes a problem in the beta globin part of hemoglobin, changing the way that hemoglobin works.
    