The RCAS-TVA system for mammalian cell infection. (A) Schematic of the RCAS retrovirus construct. This construct is transfected into DF1 chicken fibroblasts, where it is replication-competent and is produced at high titers. (B) RCAS viruses enter mammalian cells engineered to express the RCAS receptor, TVA. The reverse-transcribed retroviral genome integrates into the genome of the infected cell. TVA-negative mammalian cells are not infected by the RCAS virus.