TP53 and breast cancer: On the contrary, lower doses of the 26S proteasome inhibitor (i.e., 1 μM) do not affect breast cancer cell viability and are the minimum sufficient amount that determines the time- and dose-dependent (Fig. 1) accumulation of total polyubiquitinated species and the increase in the cellular content of p53, another transcription factor that rapidly undergoes proteasomal degradation [18].