Definition
Gene expression is the process by which the information stored in a gene is read out and used to build its product — usually a protein. A gene that is not expressed is inert: the information exists, but nothing in the cell acts on it.
Sapolsky stresses that the question worth asking is rarely whether an organism has a gene. Nearly every cell in a body carries the same full genome. What differs — between a neuron and a liver cell, between a calm moment and a stressful one — is which genes are being expressed and how strongly.
Why it matters
How it works
Expression begins with transcription, in which an enzyme copies the gene's DNA into a strand of messenger RNA. That RNA is processed and exported, then translated by ribosomes into a chain of amino acids that folds into a protein. The protein goes on to do the cell's work. At every stage — and especially at the start — regulatory machinery decides whether the process runs at all and how fast.