Most grammarians (well, this grammarian, anyway) eschew the apostrophe used for anything other than contractions and possessives. We do, however, grudgingly accept its use when there is otherwise visual confusion in reading the text. Examples are:
his's, hers's, p's and q's, etc.
We do insist that none should be used with such as ABCs, UFOs and other instances where there is no uncertainty about the function of the s.
We saw many UFOs, but only one UFO's door opened.
- For Teachers