humans are the ultimate target population, this restriction of virus and vector entry could create problems. The phenomenon is likely akin to the N/B tropism of MLV-based vectors that maps to the capsid. The HIV/SIV restriction in some monkey cells also maps to the capsid (76), and it has recently been reported that the block can be overcome by substituting the cyclophilin A-binding region in the capsid with that from macrophage tropic HIV (78). Another example of unexpected restriction of transduction familiar to this author is that we were unable to transduce muscle tissue from mouse with FIV vectors but were able to easily introduce the same vectors into guinea pig muscle (47). However, it should also be noted that this restriction is generally saturatable, and, for example, HIV vectors have been used very successfully to treat a macaque model of Parkinson's disease (79), where there is a restriction to HIV entry.

Because the knowledge of the virology of these viruses is incomplete, we should expect further unforeseen observations.

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