Heat oven to 375°F. In large bowl, mix ground beef, 1/4 cup of the ketchup, the bread crumbs, Worcestershire sauce, salt and egg. Press 1/3 of mixture in bottom of ungreased 8x4-inch loaf pan.
Place 2 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; pat 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 well 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 thermometer inserted in center of meat loaf reads 160°F. Let stand 15 minutes before cutting into slices.