Hi Caitlin - I agree with Youssef's comment above. In the context of the cell, downregulation refers to the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external stimulus. The complementary process that involves increases of such components is called upregulation.
For example, take a cancer cell. A cell that has become cancerous has developed a mutation that allows it to evade cell death. Often, this occurs when the cell has down-regulated receptors on the cell surface that signal apoptosis. In this example, down-regulation of the receptor has proven to be a beneficial response for the cell, allowing it to survive.
Hope this helps!
- Sydney