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Ultimately, we would like to be able to provide a comprehensive set of vectors. In effect, if someone were to write down the properties of a vector — which cells or tissues it would infect, which cells or tissues it would replicate in, which cells or tissues it would be expressed in, etc. — then we could supply the relevant vector. Progress has been made, but there are some problems that have not been solved. In some cases, we know how we should proceed but haven’t had sufficient time to build the right vectors. In other cases, additional research will be required, and we are not certain when, if ever, the problems will be fixed. Here is a short list of some of the things that we would like to do:

  1. Although we initially planned to produce helper cells for BBAN, we have found that complementation with VSV-G in transient transfections works quite well, and there is no replication-competent virus produced.

  2. Make an RCAS derivative that will replicate efficiently in mammalian cells. As we said, the fact that RCAS vectors cannot replicate in mammalian cells has some advantages, but also has some disadvantages. What we would really like is to have a choice: RCAS vectors that can replicate on mammalian cells as well as those that cannot. This has been a difficult problem. Although we’ve made considerable progress, we do not yet have a vector with all of the desired properties.

  3. Although MLV cannot infect nondividing cells, it was recently shown that ASLVs (including RCAS vectors) can infect nondividing cells in culture, albeit at a lower efficiency relative to infection of dividing cells (Hatziioannou and Goff, 2001; Katz et al., 2002). We are exploring the possibility that RCAS vectors may be useful for infecting nondividing cells in animal models.

  4. Create modified envelope genes that will direct the RCAS vectors to the cell surface protein of our choice. This is the holy grail of retroviral vector design. We’re working on this particularly difficult problem, as are a number of other laboratories. We hope that some of the recent advances in understanding the structure and function of retroviral envelope genes will help solve this problem.

  5. Having developed both a shuttle vector (RSVP) and a gene-trap vector (pGT-GFP), we are trying to create a shuttle-trap vector that has the advantages of both.