Included in the Dictionary only because we need a clarification on the singular and the plural. The singular is “an M&M^®.” Note that “an M” is not generally used by literate society as the singular. The plural is “M&M’s^®,” of course, and spelled that way too. It is not a possessive; however, this spelling is based on the grammatical rule that if a word ends with a single capital letter that stands for something else, such as in an acronym, then an apostrophe, in fact, and an “s,” are used to make it plural.