FLT3 and gastrointestinal stromal tumor: In this context, it is well described for TKI therapy of CML and GIST and has recently been shown for TKI therapy in acute leukemia as well, that resistance towards TK-inhibitors is often caused by secondary mutations within the tyrosine kinase domain (such as point mutations at FLT3 D835) of the respective tyrosine kinase [40].