TYK2 and gastrointestinal stromal tumor: PDGFRA gene activation was frequently found in 82% to 93% cases of gastrointestinal stromal tumors (GIST).10 And a small case of PDGFRA gene (~5%) expressed the point mutation, so PDGFRα was in a sustained activation state.10 Specifically, PDGFRA mutations are found mostly in exons 18 (tyrosine kinase 2 (TK2) domain).