Figure 2

Puro-Δtk retroviral vector constructs and titres. (a) All vectors were constructed from murine leukemia virus (MuLV) backbones and contained the Puro-Δtk cassette. pWWF1, pWWF2, and pWWF3 are three revertible pBabe-based vectors containing a loxP site in the 3' long terminal repeat (LTR) [26]. pWWF4 and pWWF5 were constructed from vectors pQCXIX and pMSCV-Neo. pWWF6 was constructed in the pRetro-Super backbone. In some of these vectors the 3' LTR has a self-inactivating mutation provided by an internal deletion to generate a self-inactivating provirus. (b) Titers of transiently produced virus on ES cells after a single round of infection with 0.1 ml viral supernatant are illustrated by staining puromycin-resistant colonies in 24-well plates.