Feline McDonough Sarcoma-like tyrosine kinase (FLT3) is a member of the class III tyrosine kinase receptor family. Ligand binding induces conformational changes in the FLT3 receptor, which facilitates its dimerization and autophosphorylation. Tyrosine-protein kinase HCK (HCK) associates with the phosphorylated Y589 and Y591 residues of FLT3. This binding results in further phosphorylation of the FLT3 receptor to make it fully active (Heiss et al. 2006, Mitina et al. 2007). There may be more unknown binding sites for HCK on FLT3.