Figure 1.

The reporter system for Cre activity. The expression cassette has the RFP gene in-frame to the ATG initiation codon with its adjacent loxP sequence. Translation of RFP is terminated at a UAG stop codon. The ORF of a humanized eGFP, lacking an initiation codon, is downstream of the distal loxP site. Following Cre-mediated excision of RFP, eGFP codons are placed in-frame of the proximal ATG-LoxP sequence and are consequently translated. The reporter cassette was inserted into the FMEV-based retroviral vector SF91, which allows stable integration in the host genome and high expression of the reporter genes (8). FMEV, Friend-MCF/MESV hybrid vectors; wPRE, woodchuck hepatitis B virus post-transcriptional regulatory element (10).