FACS analysis of ES cells bearing different Atrx alleles as shown, after costaining for Annexin V (FITC, x-axis, logarithmic scale) and propidium iodide (y-axis, logarithmic scale). The size of the early apoptotic (Annexin-positive, PIlow) and late apoptotic or necrotic (Annexin-positive, PIhi) populations is indicated for each genotype.