This lecture covers the reversible and irreversible post-translational processes that occur in the endoplasmic reticulum, including the formation of disulfide bonds, isomerization of prolines, and proteolytic cleavage. It also discusses the challenges related to cysteine and disulfide bridges, protein folding, and the role of chaperones. Examples such as the influenza virus and protein misfolding diseases are used to illustrate these concepts.