PPIB and gastrointestinal stromal tumor: 20–40% of all KIT/PDGFRA WT GIST are succinate dehydrogenase complex (SDH)-deficient GIST, recognized by the loss of subunit B (SDHB) protein expression most often due to germ-line and/or somatic loss-of-function mutations in any of the four SDH subunits (A, B, C, or D).