The endoplasmic reticulum (ER) is a special membrane structure found only in complex cells. Parts of the ER are studded with ribosomes--the cell's protein-making machinery. Proteins that require special conditions or are destined to become part of the cell membrane are processed in the ER and then handed off to another organelle called the Golgi apparatus. The Golgi functions as a cellular post office. Proteins that arrive there are sorted, packaged and transported to various destinations in the cell. Scientists are studying many aspects of the ER and Golgi apparatus, including a built-in quality control mechanism cells use to ensure that proteins are properly made before leaving the ER. NSF scientists hope someday to understand how the ER monitors abnormal proteins and destroys them in order to keep cells healthy.