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The option `--do_lower_case` is currently required by the uncased models (i.e., bert-base-uncased, bert-large-uncased).

Results:
BERT-BASE without --do_lower_case:  'exact': 73.83, 'f1': 82.22
BERT-BASE with --do_lower_case:  'exact': 81.02, 'f1': 88.34