Figure 1

Mode of tagging defines the optimal targeting cassette. Two highly potent chromosomal integration sites were screened upon random integration of an expression cassette driven by the SV40 promoter and the MSCV promoter, respectively. By means of Flp recombinase mediated cassette exchange, the screening cassette was exchanged for various expression cassettes, thereby integrating expression cassettes driven by the SV40 promoter, the adenoviral major late promoter (AdmLP) or the MPSV promoter into the same chromosomal site. The expression level upon targeting the various cassettes was determined and related to the expression level of the tagged cell.