DNA differences between people are tiny — sometimes a single letter out of billions. But a single letter can change a protein's instructions, affecting how much of the protein is made or how well it functions.
Why the same medicine can behave differently
If a DNA difference changes the protein a drug acts on, the drug may work differently depending on which version of that protein a person carries. This is one reason a medicine can help one person and fail, or cause harm, in another.
Comparing DNA across many people points to genes, and therefore proteins, that may be involved in a disease. Those proteins become candidate targets. DNA is not the medicine and not the target — it is the evidence that shows scientists where to look.