KIT and gastrointestinal stromal tumor: Likewise, our current studies confirm that ponatinib inhibits some but not all common imatinib-resistance secondary KIT mutations, with the common KIT exon 13 V654A mutation being a particular challenge.19 Although the drug screens show differential activity for each of the TKIs against various imatinib-resistance KIT mutations, the in vitro IC50 values do not permit precise prediction of clinical activity, particularly in absence of correlative studies from GIST biopsies and/or cfDNA in patients receiving these agents.