Characterization of an immortal CD56+ clone during in vitro propagation. Three time points are shown: population doubling (PD) 35, 120 and 234 (see growth curve in Figure 3C). Top row: flow cytometry histograms of CD56 stained cells (red lines) and control cells without the primary antibody (green lines, representing background fluorescence). Uniform expression of CD56 was maintained throughout >200 PDs of in vitro cultivation (percentage of CD56+ cells is indicated). Second row: cells stained for expression of the muscle-specific intermediate-filament protein, desmin (green) and counterstained with 4',6-diamidino-2-phenylindole (DAPI) (blue). Third and fourth rows: phase (third row) and fluorescence (fourth row) images of cells in differentiation medium after 3 days (PD 35) or 7 days (PD 120 and PD234) and stained for myosin heavy chain (MF20, green) and DNA (DAPI, blue).