Heat oven to 375 degrees. In bowl, mix ground beef, 1/4 cup of the ketchup, the bread crumbs, Worchestershire sauce, salt and egg. Press 1/3 mixture in bottom of ungreased 8"x4" loaf pan.
Place two pieces of the string cheese lengthwise and evenly spaced over beef mixture, with one end of the cheese about 1/2 inch from one end of pan. Cut 2 pieces of string cheese in half crosswise. Place 2 half pieces on beef mixture so cheese extends full length of pan. Press cheese into beef mixture.
Top with half of remaining beef mixture; put evenly in place. Top with remaining string cheese in 2 rows; press in. Press remaining beef mixture over cheese. Press ends and edges of beef mixture to seal in cheese; smooth top.
Bake 45 minutes. Meanwhile in small bowl, mix remaining 1/4 cup ketchup, the mustard and brown sugar.
Remove meat loaf from oven. spoon juices from meat loaf; spoon ketchup mixture evenly over top,
Return to oven; bake 30 to 45 minutes longer or until meat thermometer inserted in center reads 160 degrees. Let stand 15 minutes before cutting into slices.