CHO cells transfectants, cultivated on glass lamellae, were fixed by the addition of 2% formaldehyde in a culture medium for 10 min. After washing in PBS, the cell monolayer was incubated with the anti-A mAb 3-3A at 0.5 μg/mL in PBS for 1 h and washed thrice in PBS before incubation for 30 min with TRITC-labeled anti-mouse IgG (Sigma) diluted at 1/400. After three final washings in PBS, slides were mounted in Mowiol and observed under a Leica TCS SP (Heidelberg, Germany) confocal fluorescence microscope. Negative controls were incubated with the secondary antibody alone.