A gene does not do work by itself. It carries the instructions for building a protein, and proteins are the molecules that actually carry out tasks in the body: they speed up chemical reactions, move substances around, send signals between cells, and build structures.
The path runs in one direction. A gene is read, and its instructions are used to assemble a protein. That protein then performs a job. When a protein behaves in a way that drives a disease — producing too much of a signal, or failing to do its normal job — it becomes an attractive place for a medicine to intervene. That place is called a drug target.
Proteins are the most common drug targets for a practical reason: they have shapes. A protein is folded into a specific three-dimensional form with pockets and surfaces, and a small drug molecule can be designed to fit into one of those pockets and change what the protein does. DNA itself is rarely the direct target of a medicine; it is the source of the instructions, while the protein is the working part that a drug can physically grab hold of.