Expression systems are biological tools used to produce proteins, peptides, or other molecules in a controlled environment. These systems utilize living cells, such as bacteria, yeast, or mammalian cells, to translate genetic information into functional products. By inserting specific genes into these cells, researchers can harness their machinery to create large quantities of desired substances for research, pharmaceuticals, or industrial applications. Different types of expression systems are chosen based on the requirements of the target protein, such as its size, complexity, and post-translational modifications. Common examples include Escherichia coli for simple proteins, Saccharomyces cerevisiae for yeast-based production, and mammalian cell lines for more complex proteins that require specific modifications.