To each well of a 96-well plate, containing 10.000 CD4 + HeLa-P4 cells inhibitor and control proteins were added at various concentrations and the cells were incubated for 48 h. Thereafter the medium was changed and 10 μl MTT (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) solution (5 mg/ml MTT dissolved in PBS pH 7.4) was added. The plates were incubated at 37°C for 4 h after which the medium was removed and 100 μl of lysis buffer (9.94 ml DMSO, 60 μl HCl, 1 g SDS) added to each well. The plates were incubated for 10 min at 37°C. 