Scientific papers tend to contain lots of abbreviations—acronyms, initialisms,* gene symbols and protein designations, element and isotope symbols, chemical formulas, and so on—and authors sometimes have difficulty choosing the correct indefinite article (“a” or “an”) to use with abbreviations. The general rule is that the choice depends on how the abbreviation would be pronounced if read aloud: if the pronunciation starts with a vowel sound, use “an,” and if it starts with a consonant sound, use “a.” Let’s look at some examples:
The UTR lengths were calculated as the number of reads in a UTR divided by the number of reads in the CDS multiplied by the CDS length.
Here “UTR,” pronounced letter-by-letter, starts with a consonant sound (\y\ as y in yet), so “a” is the best choice. Note, however, that a Google Scholar search of “an UTR” turns up a number of hits (about 1/6 as many as “a UTR”): for example,
The blocked copy can then be replaced with an UTR-specific primer pair.
My guess is that this author read this sentence as if the abbreviation were spelled out—“with an untranslated region–specific primer pair”†—and thus I would argue that “an” is not incorrect, as long as it is used consistently throughout the manuscript.
Here are two example sentences with abbreviations pronounced as words:
Therefore, calcium ion–dependent glutamate release from astrocytes is a SNARE protein–dependent process that requires the presence of functional vesicle–associated proteins.
All the antisera reacted with p240 in an HDAC-specific manner
“SNARE” is pronounced with an initial consonant sound and thus gets “a,” and “HDAC” starts with the vowel sound “aitch” (“aitch-dak”) and thus gets “an.”
Some abbreviations are pronounced as words by some people and letter-by-letter by others; SNP (for single nucleotide polymorphism) is an example, and you’ll find both “a” and “an” used in the literature:
Here we report the results of an SNP [pronounced “s-n-p”] survey of 21 maize loci.
The most common cause of the loss of hepatic CYP3A5 expression is a SNP [pronounced “snip”] at nt 22,893 in intron 3 of CYP3A5*3.
The same is true for some gene symbols:
KmycJ are K562 cells with a MYC gene [pronounced “mik gene”] inducible by ZnSO4.
The cells show L3 morphology. . .with coexpression of TdT and surface light chains in addition to an MYC gene [pronounced “m-y-c gene”] translocation.
I would argue that both are correct; just be consistent throughout your manuscript.
What about element symbols? The ACS Style Guide states that element names are pronounced even when element symbols are used, and therefore the choice of article depends on the pronunciation of the element name:
The analyzed DNA is hybridized with a primer nucleic acid that is associated with a Au surface [pronounced “a gold surface” not “an a-u surface”].
TGA-DSC measurements were performed under a He flow [pronounced “a helium flow” not “an h-e flow”].
The same guidance applies to simple chemical formulas:
Indomethacin, a nonsteroidal inhibitor of prostaglandin synthetase, was diluted in a Na2CO3 buffer [pronounced “sodium carbonate buffer”].
Isotopes are treated differently. The ACS Style Guide calls for the element symbol to be pronounced before the number (e.g., “14C” is pronounced “c fourteen”), and thus the pronunciation of the element symbol should determine the choice of article:
As N-1 becomes pyramidal, an 15N [pronounced “en fifteen”] isotope effect of up to 2–3% is observed.
Note, however, that this “rule” does not appear to be followed universally. I found that “a 15N isotope” is much more common than “an 15N isotope”: for example,
Reaction of hydroxide ion with the neutral phosphotriester exhibits a 15N-isotope effect consistent with only 25% bond fission
Perhaps most people read this as if the element name were spelled out—“a nitrogen-15 isotope”—in which case “a” is in fact correct. Again, make a choice and stick to it consistently.
*Acronyms are abbreviations that are pronounced as words (e.g., ANOVA, SNARE, GABA), and initialisms are abbreviations that are pronounced letter-by-letter (e.g., DNA, UTR, EDTA).
†Or he or she speaks a language (e.g., German or French) in which the letter “u” is pronounced starting with a vowel sound.
7 replies on ““A” or “An” with abbreviations? It depends.”
It grates on my nerves when TV news people say that something is concerning. When did this start to happen?
Hi! Thank for your post. I still have a question: I have an abbreviation in a paper of mine, I abbreviated for reasons of word count limit, but would like the abbreviation to be read in the long form by the reader. Should I use “a” /”an” as it was written down in long form? Thanks! Damian
Hi, Damian. It depends on the abbreviation and what you mean by “read in long form.” Normally, custom (not writer’s preference) determines how abbreviations and acronyms are read/pronounced by readers. As an example, let’s use NATO. It’s an acronym — that is, it’s always pronounced as if it were a word (nā-tō) rather than as the individual letters: N-A-T-O. Although you might be able to get readers to read it in its spelled out form (North Atlantic Treaty Organization), you’d have a hard time getting them to read it as letters simply by using “an” instead of “a”: that is, most readers will simply see “an NATO coalition” as an error rather than read it as “an N-A-T-O coalition.”
You might have better luck with an abbreviation, such as NMR, which is pronounced as the letters, N-M-R. Normal usage calls for “a nuclear magnetic resonance spectrometer” and “an NMR spectrometer, but if you used “a NMR spectrometer,” that might encourage readers to read it as “a nuclear magnetic resonance spectrometer,” but, again, some readers will simply see “a NMR spectrometer” as an error.
Perhaps I could give you a more satisfactory answer if you told me what your abbreviation is.
Thank you for your post. I still have a doubt of a or an in the abbreviation of N.C.C…..whether it’s a NCC or an NCC……e.g. My brother was……N.C.C. cadet. I need your help. Thank you so much.
It depends. If NCC is usually pronounced as the letters, then it should be “My brother was an NCC cadet.” However, if NCC is usually pronounced as National Cadet Corps, then it should be “My brother was a NCC cadet.”
Hi,
What about LoD -Level of Development? Should I use a or an before it or maybe the?
It depends. If “LoD” is usually read as “level of development,” then use “a.” If it is usually read as the letters “L-o-D,” then use “an.” If it can be read either way, then just be consistent about using either “a” or “an” in any given document. “The” would also be correct in certain contexts.