Asterisk '*’ indicates that the combination index values are means of four experiments. They were derived from median effect analysis, according to independent action model, for three different levels of toxicity and three alternative schedules of administration. CI values < 1 indicate synergy and values > 1 indicate partial antagonism. Administration schedules were simultaneous, bortezomib 24 h before X-radiation (BZ before) and bortezomib 24 h after X-radiation (BZ after). Italicised values indicate synergy.