You can prevent AutoCorrect from correcting specific abbreviations or words that have mixed capital letters and lowercase letters.
Choose, and then click the tab.
Do one of the following:
Type the abbreviation followed by a period in the Abbreviations (no subsequent capital) box and click New.
Type the word in the Words with TWo INitial CApitals box and click New.
To quickly undo an AutoCorrect replacement, press Command Ctrl+Z. This also adds the word or abbreviation that you typed to the AutoCorrect exceptions list.