Cells from MEL cells (4 × 107) or fetal liver cells from three 15.5-dpc WT mice were treated with 1% formaldehyde for 10 min at 37 °C, rinsed in ice-cold 1× Hanks' balanced salt solution with 0.1% EDTA containing protease inhibitors, collected by centrifugation at 4 °C, resuspended in a SDS lysis buffer containing protease inhibitors, and incubated on ice for 10 min.