Retroviruses are naturally occurring viruses with an RNA genome. A retroviral vector is a modified infectious virus derived from murine or avian species that can be used to introduce nonviral DNAs into a target cell. There are two prominent features of retroviralmediated DNA transduction. First, retroviral vectors can stably integrate into the host genome as a provirus. Second, retroviralmediated DNA integration requires host cell proliferation. Because retroviral vectors allow stable and efficient DNA transduction in vitro and in vivo, they have been widely used in developmental studies (1-3) and certain types of gene therapy approaches (4).