English: Some serpins can spontaneously convert to an inactive latent state. The serpin PAI-1 remains in the active conformation when bound to vitronectin (green). However in the absence of vitronectin, PAI-1 can convert to the inactive latent state. The uncleaved RCL (blue, disordered regions as dashed lines) inserts into the A-sheet and the pulls the first β-strand off the C-sheet (yellow). (PDB: 1OC0​, 1DVM, 1LJ5​​)