Genes contain the code to produce proteins, and often genes and their matching proteins are referred to by the same abbreviation. For example, the BRCA1 gene contains the code to make the BRCA1 protein.
Sometimes that can get confusing, especially in certain areas of research, such as biochemistry or cell biology, that study both proteins and genes. When you read about research like that, occasionally it can be difficult to work out whether they’re talking about the gene or the protein. Writing one of the names in italics helps to distinguish the two.
The convention became to write the gene symbol (or that of the corresponding cDNA or RNA) in italics, but keep the protein name unitalicised. That way you can more easily distinguish the BRCA1 gene and the BRCA1 protein when they’re both in the same sentence.
This practice started long ago, and it’s difficult to trace to the exact history. The American Medical Association (AMA) has included it in their Manual of Style, so all publications that adhere to this style guide for writing about genetics will italicise gene symbols.
Style guides are used by magazines, newspapers and journals to make sure that all their editors use the same rules to format all of their articles. That consistency makes it easier to read articles. A similar type of guideline, found in many style guides, says that titles of publications should be italicised. When you read a sentence that starts “An article published in Blood this week…” the use of italics here signals that it’s describing a journal called Blood, and not an article written in blood. The same is true for the distinction between genes and proteins within the same text.
When did we start italicising gene symbols?
Rules in style guides are not invented out of the blue. They are based on what is common practice.
When Rhesus blood groups were discovered in the 1940s, it was already conventional to use italics for genes, as described in an article in the British Journal for the History of Science. “Following the conventions for genetic nomenclature at the time, Landsteiner and Wiener denoted alleles using italicized two-letter symbols (Rh)”
More recent formal guidelines specific to italicising human genes often cite the HGNC guidelines from 2002. These are an updated version of a document published in 1987, which was in turn adapted from guidelines published after a meeting in 1979.
Italicising gene names is a common convention not just for human genes, but for other species as well. Even though different organisms have different rules for gene nomenclature, they all use italics.
Exceptions to the guidelines
There are exceptions to the style guidelines: For example, if you’re publishing a long list of gene symbols, and it’s obvious that every item in the list is a gene (and not a protein), they don’t all have to be in italics.
Ultimately, the convention to italicise gene symbols was put in place to help reduce confusion, and to make text easier to read.