Dear Editor,Prototype foamy virus (PFV) belongs to the genus Spumavirus in the Spumaretrovirinae subfamily of Retroviridae.Although PFV and HIV have much in common,research into PFV has lagged far behind that into HIV,as PFV appeared to be non-pathogenic both in accidentally infected humans and in experimentally infected animals.In recent decades,however,more attention has been focused on PFV because it seems to be a promising candidate vector for gene therapy in clinical applications.Compared with other retroviral vectors,vectors derived from PFV appear to be safer (Sastry L,et al.,2005).In addition,it was found that PFV vectors are efficient vehicles for stable gene transfer in proliferating cells,and that the transferred genes are able to achieve stable integration and long-term expression in the target cells.