We use an apostrophe to create plural forms in two limited situations: for pluralized letters of the alphabet and when we are trying to create the plural form of a word that refers to the word itself. Here we also should italicize this "word as word," but not the 's ending that belongs to it. Do not use the apostrophe+s to create the plural of acronyms (pronounceable abbreviations such as laser and IRA and URL*) and other abbreviations. (A possible exception to this last rule is an acronym that ends in "S": "We filed four NOS's in that folder.")