ESR1 and breast cancer: In the ER+/HER2- cluster (n = 452), which encompasses both Luminal A (n = 167) and Luminal B HER2- (n = 285) breast cancers, a significant association of the MMR status according to PTEN IHC was observed (p < 0.0001, Fisher’s exact test) (Table 1 and Table 5).