Senescent MEFs stain more intensely with histone variant macroH2A. (a) Human IMR90 fibroblasts transduced with either control or H-RasG12V were stained for histone variant macroH2A and DNA (DAPI). Inactive X chromosome is enriched in macroH2A in female cells (arrow). (b) Control or H-RasG12V-transduced senescent MEFs were stained for histone variant macroH2A. Results were quantitated using metamorph and expressed as histograms and box plots: The red line is the median (50th percentile); the box itself encompasses the 25th and 75th percentiles (Inter Quartile Range (IQR)); the whiskers are the most extreme data points within 1.5 × IQR; crosses outside the whiskers are outliers. (c) Images of cells stained as in (b). White arrows indicate a cell with increased macroH2A stain.