The first viral vector developed for gene delivery was derived from the Moloney murine leukemia (MLV) γ-retrovirus. Gamma-retroviruses are capable of reverse transcribing their single stranded RNA genome into double stranded DNA, which is then stably integrated into dividing host cells. This integration results in long-term transgene expression. The γ-retroviral vector is widely used in ex vivo gene therapy applications (involving hematopoietic stem cell (HSC) and immune cell genetic engineering) for the treatment of genetic diseases and cancer.