Determination of the amount of ATM in resting and PHA-stimulated PBMCs. (A) Immunofluorescence labelling of the 2 cell types with ATM-4BA antibody. DAPI was used to stain nuclei. (B) Plastic-adherent cells (resting) isolated from human PBMC were enriched for monocytes (R2, CD33⁺) and contained some T lymphocytes (R1, CD33-) as shown by FACS analysis. Cells were labelled either with ATM-4BA or with preimmune rabbit sera as a control. The MFI for each population is shown in brackets.