2.6.1. Preparation of Substrate Cells

HeLa cells were grown to 80% confluence, washed with PBS and treated with 1 μM Ro plus 4 min of light. Then they were washed with cold PBS, detached by trypsinization, centrifuged for 5 min at 800× *g* at 4 °C and resuspended in freezing medium (DMEM supplemented with 20% fetal bovine serum, and 10% DMSO) at a concentration of 1 × 106 cells per mL. Aliquots of 0.7 mL were frozen slowly and stored at −80 °C.
