First, viral pseudotyping with a foreign glycoprotein can occur easily in many cases although there are some exceptions, one of which is the exclusion of HIV-1 or SIV Env with the long CT from most retrovirus cores [70] . With respect to HIV-1, the virus can be pseudotyped with Env glycoproteins not only from several other retroviruses but also with those from other virus families such as ortho (para) myxoviruses and flaviviruses [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] .