1/4 cup cider vinegar
3 tablespoons prepared coarse−ground mustard
3 cloves garlic, peeled and minced
1 lime, juiced
1/2 lemon, juiced
1/2 cup brown sugar
1−1/2 teaspoons salt
6 tablespoons olive oil
ground black pepper to taste
6 boneless, skinless chicken breast halves

In a large, nonreactive container, thoroughly mix cider vinegar, whole grain mustard, garlic, lime juice, lemon juice, brown sugar, and salt. Whisk in olive oil and pepper. Place chicken in the mixture. Cover, and marinate chicken in the refrigerator 8 hours, or overnight. Preheat an outdoor grill for high heat, and lightly oil grate. Place chicken on the prepared grill, and cook 10 minutes per side, until no longer pink and juices run clear.
Discard remaining marinade.