MLH1 and breast cancer: In the meantime, dysregulation of the MutL mismatch repair complex (MLH1/3, PMS1/2), including MLH1 promoter hypermethylation, is linked to endocrine therapy and chemotherapy resistance in ER+ breast cancer (Haricharan et al., 2017; Dasgupta et al., 2019), suggesting that MLH1 hypermethylation may correlate with poor prognosis of ER+ breast cancer.