Puromycin labelling for measuring the intensity of translation was performed as previously described [47] . For immunoblots, 10 mg/ml puromycin (Sigma, min 98% TLC, cell culture tested, P8833, diluted in PBS) was added in the culture medium and the cells were incubated for 10 min at 37uC and 5% CO 2 . Where indicated, 25 mM cycloheximide (Sigma) was added 5 min before puromycin. Cells were then harvested, centrifuged at 4uC and washed with cold PBS prior to cell lysis and immunoblotting with the 12D10 antibody.