(B) FACS profiles of ES cells bearing either functional (Atrx WT or Atrx flox) or null (Atrx Δ18Δneo) Atrx alleles stained for the mitosis marker phosphorylated (Ser10) histone H3 (FITC, y-axis, logarithmic scale) against propidium iodide (x-axis, linear scale). The size of the mitotic population (phosphoH3S10-positive, PIhi)(gate R3) is indicated for each profile. The ES cell clone analysed in each trace is indicated.