The endoplasmic reticulum (or ER) is a complex cytoplasmic membrane structure presented in eukaryotic cells. The ER is involved in protein folding, lipid synthesis and trafficking, and regulation of the intracellular calcium balance. Disturbances in the normal functions of the ER lead to the ER stress. Usually, it is caused by the accumulation of misfolded proteins. However, the ER stress is also associated with atherosclerosis and may be caused by modified LDL.