To analyze whether or not growth of triple-negative breast cancer (TNBC) or progesterone receptor-positive breast cancer (PR+BC) patient-derived xenografts (PDXs) causes oxidative stress, we evaluated the levels of 4-HNE in the hippocampal tissues of tumor-bearing mice (Vila et al., 2008; Zheng et al., 2014). Here, PGR is linked to breast cancer.