In the beginning of a sentence, should I capitalize abbreviations such as the following:

  • hPSC (human pluripotent stem cell)
  • mESC (mouse embryonic stem cell)
  • rDNA (recombinant deoxyribonucleic acid)

I have seen both lower and upper case for the two first, while rDNA always seems to be in lower case. I'm curious which is the linguistically correct form.

  • Wikipedia, at least, seems to use the lowercase letter at the beginning of a sentence, for example see the article on mRNA: en.wikipedia.org/wiki/MRNA
    – jbeldock
    Apr 8, 2013 at 23:46
  • 2
    Apparently, the Chicago Manual of Style's 16th edition includes a new provision permitting sentences beginning with a brand name (e.g. iPad, iPod) which starts with a lowercase letter to retain the lowercase letter: chicagomanualofstyle.org/about16_rules.html. This might be analogous.
    – jbeldock
    Apr 8, 2013 at 23:51
  • Yeah, I use the same approach as Wikipedia on this one. I am, however, curious if there exist a correct and incorrect approach. I don't think the iPad rule applies since these are abbreviations rather than names, but it's an interesting one nonetheless. Apr 8, 2013 at 23:58
  • We also see this in mathematics. Do not begin a sentence with a variable like x . Instead, re-word. Some extra meaningless word may be used. "Now x is..." or "Then x is..." or "Note x is..."
    – GEdgar
    Apr 15, 2019 at 12:24
  • 1
    This is a style issue, rather than "correctness". If you are writing for someone else (eg. a journal, then check their guidelines). If there are no such guidelines, then choose your style and be consistent.
    – user323578
    Apr 15, 2019 at 12:28

3 Answers 3


Whenever you encounter a situation where a "rule" (such as begin a sentence with an upper case letter) will reduce the readability of your test (Is MESC the same as mESC?) then rewrite. Do not try to find a loophole or a special rule that will need to be explained or justified. The meaning of your text is paramount.

There is a reason why chemistry texts do not begin a sentence about acids and bases with pH.

  • 1
    I side with your advice more than my own accepted answer, i.e. do away with the ambiguity altogether! Apr 19, 2013 at 20:32
  • 1
    Revisiting my question, I also prefer this advice as it explains the reasoning behind the rules of acheong87's answer. Jun 19, 2013 at 3:21

According to Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers (C B E Style Manual),

Even if the editor chooses a style that calls for initial capitalization of every term, some terms should retain an initial lowercase letter. [...] 3 A standard symbol or abbreviation that begins with a lowercase letter (pH, pK', mRNA).

Also, according to apsstylemanual.org,

An abbreviation that begins with a lowercase letter, or a term that must remain lowercase should not be changed to all caps when it begins a sentence; it should not be expanded. The word following should be is lowercase unless it is a proper noun or another acronym. If possible, reword the sentence so that it does not begin with the lowercase term

I don't see anything (other than the one regarding brand names) in the Chicago Manual of Style.


Because the original post is speaking of abbreviations, I recommend following the rule to spell out the word. For example, mRNA could become "Messenger RNA..."

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