PPARGC1B and breast cancer: In line with the downregulation of PPARGC1B expression by XBP1, we found a negative relationship between the expression of XBP1 transcript levels and mRNA levels of PPARGC1B (significance of correlation: R-value = −0.418, p-value = 9.9 × 10−48, T-value = −15.245, degrees of freedom = 1095) in breast cancer dataset of TCGA (Figure 4E).