function mutation(arr) {
  // Example

  var letter1 = arr[0].split('');
  var letter2 = arr[1].split('');


  var test ;
  for (var k = 0; k < letter2.length ; k++){

    if(letter1.indexOf(letter2[k]) > -1){
      test = true;
    }else{
      return false;
    }

   }

  return test;

}