Methods and compositions are provided for delivering a polynucleotide encoding a gene of interest to a target cell using a virus. The virus envelope comprises a cell-specific binding determinant that recognizes and binds to a component on the target cell surface, leading to endocytosis of the virus. A separate fusogenic molecule is also present on the envelope and facilitates delivery of the polynucleotide across the membrane and into the cytosol of the target cell. The methods and related compositions can be used for treating patients having suffering from a wide range of conditions, including infection, such as HIV; cancers, such as non-Hodgkin's lymphoma and breast cancer; and hematological disorders, such as severe combined immunodeficiency.