Since MuLE lentiviruses can be used to model undifferentiated pleomorphic sarcomas in vivo [24], a tumour of mesenchymal origin [22], concentrated ecotropic MuLE lentiviruses which cause either Cdkn2a silencing together with activation of HrasG12V or Pten or Trp53 silencing alone, were injected into the uterus of 4-week-old SCID/beige mice (Fig 4A). This evidence concerns the gene PTEN and sarcoma.