To provide a marker for detecting endoplasmic reticulum stress, thereby providing a marker useful for diagnosis and treatment of diseases associated with endoplasmic reticulum stress, and provide a screening method for a disease therapeutic agent to reduce endoplasmic reticulum stress.SOLUTION: The present invention provides a peptide comprising a remainder of ATF6 or OASIS family protein that has been cut on its C terminal side and N terminal side with Site-1 protease and Site-2 protease.SELECTED DRAWING: Figure 1
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