(B) Paraffin sections of wild-type or Atrxnull 7.5 dpc embryos were stained with an antibody against the mitosis marker phosphorylated (Ser10) histone H3. Sections were counterstained with haematoxylin. For both (A) and (B), the presence or absence of Atrx in each embryo was determined by staining adjacent sections with the anti-ATRX antibody (H-300) as in Figure 5 (unpublished data).