Repressors are proteins that play a crucial role in regulating gene expression. They bind to specific regions of DNA, preventing the transcription of certain genes into messenger RNA. By inhibiting this process, repressors help control when and how much of a particular protein is produced in a cell. These proteins are essential for maintaining cellular functions and responding to environmental changes. For example, in the lac operon system of E. coli, a repressor protein prevents the expression of genes involved in lactose metabolism when lactose is not present. This ensures that resources are used efficiently within the cell.