Murine leukemia virus (MLV) can be pseudotyped with a variant of the HIV envelope gene encoding the surface glycoprotein gp120-SU and a carboxyl-terminally truncated transmembrane (TM) protein, with only seven cytoplasmic amino acids. MLV/HIV pseudotyped retroviral vectors selectively target human CD4+ cells and can be used as tools to study entry of HIV into cells. Mouse T-cells are immune to HIV infection, which is primarily caused by the weak binding affinity of HIV gp120 to the murine CD4 receptor. Here we show that expression of the human CD4 receptor in murine T-cells is sufficient for syncytia formation with HIV-1 envelope expressing cells and entry of MLV/HIV pseudotyped retroviral vectors. This implies that the murine CXCR4 receptor is a functional coreceptor for MLV/HIV pseudotyped vectors and confirms previous data that the inability of HIV to replicate in murine T-cells is due to a post entry block.