The HTX (a subclone of HT1080), 293T, 293, 293T/GFP (293T cell line stably expressing GFP) and 293/GP-LAPSN (293 cells stably expressing MoMLV Gag-Pol and alkaline phosphatase or AP) cell lines have been previously described [19, 65] . The 293/ XPR1, HTX/XPR1 and CHO/XPR1 cell lines were generated by transducing the 293, HTX or CHO cells using a retroviral vector, LXSN, encoding the XPR1 receptor (LhXPR1SN, kind gift of Dusty Miller) [33] and bearing VSV-G. Infected cells were selected using G418 (Invitrogen, Carlsbad, CA) for ,10 days. All cell lines were cultured in DMEM (Invitrogen) supplemented with 10% fetal bovine serum (FBS) at 37uC at 10% CO 2 -air atmosphere at 100% relative humidity.