BTK and gastrointestinal stromal tumor: In comparison to GIST‐derived exosomes, huMC KIT‐EVs uniquely contain proteins distinct for mast cell functions, including ADGRE2 (EMR2), FCGR2, HPGD, L1CAM, SYK, BTK, CSK, PLCB4, LAT, LAT2, GRB2, TRPV2 and CACNA2D2 (Table S2).