The ecotropic receptor may serve as such a factor since more Neor colonies were obtained when the pNCAIRES-GFP virus and the VLPs (transducing the Neor vector) had different (the ecotropic and the VSV-G glycoprotein, respectively), instead of the same, envelopes, resulting in the use of different receptors to initiate infection. Here, THBS1 is linked to infection.