THE BOEING 767 WAS HOLDING AT FL240, BETWEEN CLOUD LAYERS, IN AN AREA OF KNOWN THUNDERSTORMS. THE PASSENGERS WERE SEATED AND THE SEAT BELT SIGN WAS ILLUMINATED. THE FLIGHT ATTENDANTS WERE PREPARING THE CABIN FOR LANDING, WHEN THE AIRPLANE ENCOUNTERED TURBULENCE. A FLIGHT ATTENDANT WORKING IN THE FORWARD CABIN WAS THROWN IN THE AIR; HE SUBSEQUENTLY RECEIVED FRACTURES OF BOTH ANKLES. THREE OTHER FLIGHT ATTENDANTS RECEIVED MINOR INJURIES.