In HIV, pseudotyping is used to change the tropism of the virion.  In other words, pseudotyping changes the way the Lentivector targets a specific cell for infection.  In wild type HIV-1, the envelope protein recognizes and binds CD4, a molecule present of the surface of helper T cells and other immune cells.  Most often, the HIV envelope protein is replaced with vesicular stomatitis virus glycoprotein-g (VSV-G).  It is commonly used because it brings stability to the lentivector allowing it to be ultracentrifuged, but also because its phospholipid receptor is universally expressed in mammalian cells.  Due to its broad tropism, VSV-G is a good choice for pseudotyping HIV lentivectors.  However, lentivectors can be pseudotyped to infect specific cells or cell types. Using glycoproteins that have select tropism could be used to infect specific targets.  For instance, glycoproteins derived from viruses that cause lung infection could be useful for gene therapy of the human airway.