ATF4 and neoplasm: Notably, in chondrosarcoma, only a single study reports that PRP-1 (proline-rich polypeptide), a toll-like receptor ligand, increases the expression of PERK, eIF2α, ATF4, CHOP, ATF6, IRE1α, and XBP1 [124]; however, no studies have yet investigated the functional role of the UPR in this tumor.