SSEA-4 levels in established human MSCs. Flow cytometry analyses of SSEA-4 expression in (A) standardized preparations of mesenchymal stem cells obtained from the Tulane Center for Gene Therapy at different passages and (B) reference controls for levels of expression; EC cell lines, N-TERA-2 and TERA-1 and 3T3 fibroblasts. Fluorescence intensity of SSEA-4 is observed in the x-axis. Percentages represent the fraction of cells that express SSEA-4. The y-axis represents PE, which provides a measure of autofluorescence.