Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Kidney Cysts ?
    ### Response:
    A cyst is a fluid-filled sac. There are two types of kidney cysts.     Polycystic kidney disease (PKD) runs in families. In PKD, the cysts take the place of the normal tissue. They enlarge the kidneys and make them work poorly, leading to kidney failure. When PKD causes kidneys to fail - which usually happens after many years - people need dialysis or kidney transplantation.
    