Comparison of superoxide production in bone marrow– and ES cell–derived neutrophils in response to 10 ng/mL PMA Amplex Red (Molecular Probes) was used to assess the production of hydrogen peroxide, which is a reflection of superoxide production. For this assay, 35 000 Amplex Red–loaded cells per well of a low adherence 96-well plate were used as described in “Materials and methods.” Error values were too low to resolve.