Genetic errors may result in abnormal synthesis of proteins, abnormal folding and processing of proteins after they are synthesized, or changes in the functional properties of proteins. Although the synthesis of proteins begins in the cytoplasm, many are destined to be expressed on the cell surface (e.g., surface-recognition molecules, ion channels, receptors, and adhesion molecules) or secreted (e.g., hormones, growth factors, extracellular matrix proteins, and proteolytic enzymes). Newly synthesized membrane and secretory proteins are transported into the endoplasmic reticulum, the internal membranous network of the cell, in a largely unfolded state. When they leave the endoplasmic reticulum to move through . . .