While most people understand that genes contain the DNA “instructions” for how any organism is put together and operates, those genes are not static instructions like you might find in a car manual. Instead, the gene contains information that describes how to create proteins or other molecules that affect how cells in the body function. A single gene can contain different sets of instructions that affect which protein or molecule is produced and what its function is. Gene expression refers to what those instructions are and how they are used, including what protein is produced. Gene expression can change depending on the environment in the cell, whether it’s limited to that cell or caused by larger processes outside a person’s body (such as environmental exposures like pollution).
Deeper dive
The easiest way to think of gene expression is like an on/off switch with a volume control: expression of the gene determines which proteins are made, when they’re made and how many are made. Protein production relies on two processes, transcription (when DNA is copied to create messenger RNA) and translation (when messenger RNA is used to carry the instructions from the DNA to the cell’s ribosomes, where proteins are made), nicely and simply described here. Only a tiny proportion of genes are being expressed at any given point in a cell — the ribosomes can only make so many proteins at a time. To read more in depth about gene expression and regulation, see this Nature Education article.