ER stress, or endoplasmic reticulum stress, occurs when there is an imbalance between the protein folding capacity of the endoplasmic reticulum (ER) and the demand for protein folding. This can lead to the accumulation of misfolded or unfolded proteins within the ER, causing cellular dysfunction and triggering cellular signaling pathways collectively known as the unfolded protein response (UPR). ER stress has been implicated in a variety of diseases, including neurodegenerative disorders, diabetes, and cardiovascular diseases. Researchers study ER stress to better understand its underlying mechanisms and develop targeted therapies to alleviate its effects on cellular function.