Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) progressive familial heart block ?
    ### Response:
    Progressive familial heart block is a genetic condition that alters the normal beating of the heart. A normal heartbeat is controlled by electrical signals that move through the heart in a highly coordinated way. These signals begin in a specialized cluster of cells called the sinoatrial node (the heart's natural pacemaker) located in the heart's upper chambers (the atria).
    