A PEST sequence is a peptide sequence that is rich in proline (P), glutamic acid (E), serine (S) and threonine (T). It is associated with proteins that have a short intracellular half-life, so might act as a signal peptide for protein degradation. [1] This may be mediated via the proteasome [2] [3] or calpain. [4] It has implications in cancer research.