PSMB1 and cancer: Interestingly, MMP11 overexpression in the PyMT cancer model led to an abnormal proteasomal activity as shown by decreased expression of genes encoding the proteasomal subunits Psmb1 and Psmd1 in 6-week-old PyMTTg; MMP11Tg mice as compared to control animals (Figure 5Cb), suggesting that an accumulation of misfolded proteins could be responsible for the impaired mitochondrial function.