Paramyxoviruses include many important animal and human pathogens. Most paramyxoviruses have two integral membrane proteins: fusion protein (F) and attachment proteins hemagglutinin， hemagglutinin-neuraminidase， or glycoprotein (G)， which are critical for viral entry into cells. J paramyxovirus (JPV) encodes four integral membrane proteins: F， G， SH， and transmembrane (TM). The function of TM is not known. In this work， we have generated a viable JPV lacking TM (JPV∆TM). JPV∆TM formed opaque plaques compared with JPV. Quantitative syncytia assays showed that JPV∆TM was defective in promoting cell-to-cell fusion (i.e.， syncytia formation) compared with JPV. Furthermore， cells separately expressing F， G， TM， or F plus G did not form syncytia whereas cells expressing F plus TM formed some syncytia. However， syncytia formation was much greater with coexpression of F， G， and TM. Biochemical analysis indicates that F， G， and TM interact with each other. A small hydrophobic region in the TM ectodomain from amino acid residues 118 to 132， the hydrophobic loop (HL)， was important for syncytial promotion， suggesting that the TM HL region plays a critical role in cell-to-cell fusion.