English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
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 '13 at 23:46
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 '13 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. – cheflo Apr 8 '13 at 23:58
up vote 1 down vote accepted

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.

share|improve this answer
I side with your advice more than my own accepted answer, i.e. do away with the ambiguity altogether! – Andrew Cheong Apr 19 '13 at 20:32
Revisiting my question, I also prefer this advice as it explains the reasoning behind the rules of acheong87's answer. – cheflo Jun 19 '13 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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.