Internal tandem duplication (ITD) of the juxtamembrane (JM) domain of FLT3 is the most frequent mutation in human acute myeloid leukemia, and is significantly associated with leukocytosis and a poor prognosis. Previously we reported that FLT3 with ITD (FLT3/ITD) formed a homodimer and was autophosphorylated on tyrosine residues, while the mechanism remains unclear. In this study, we elucidated the role of the JM domain in FLT3 activation. Mutant FLT3 with not only ITD but also an elongating or shortening JM domain transformed murine IL3-dependent myeloid progenitor cell line 32D regardless of the tyrosine residues in the JM domain. These mutant FLT3s were constitutively tyrosine phosphorylated and activated signal-transduction molecules such as SHC, MAP kinase and STAT5a. Notably, co-transfection of the truncated FLT3/ITD lacking kinase and C-terminal domains with the wild type (Wt)-FLT3 into 32D cells resulted in the autonomous proliferation. In these cells, truncated FLT3/ITD generated a hetero-complex with Wt-FLT3 and Wt-FLT3 was constitutively tyrosine phosphorylated. These findings indicate that the FLT3 JM domain plays an important role in receptor activation, and that the length-mutated JM domain induces ligand-independent receptor activation but also activates Wt-FLT3 in a trans-manner.