Schematic structure of the FLT3 receptor consisting of 5 immunoglobulin-like folds that make up the ligand-binding extracellular domain (ED), a single transmembrane domain (TD), and a cytoplasmic domain containing the juxtamembrane domain (JMD) as well as the kinase domain divided into 2 parts by a short kinase insert (KI). Of 282 ITDs analyzed in our series, 69.5% (n = 196) of the ITDs had the insertion site in the JMD, whereas 30.5% (n = 86) of the ITDs were localized in the first tyrosine kinase domain (TKD1) of FLT3. ITD indicates internal tandem duplication; TKD1, tyrosine kinase domain-1; and TKD2, tyrosine kinase domain-2.