The results of a meta-analysis of 27 studies and 10,231 cases performed by Li et al. [50] showed that there seems to be an association between PTEN expression loss and tumour aggressiveness in breast cancer patients, especially those with ER, PR, and TNBC tumours, thus demonstrating the involvement of PTEN in the initiation and malignancy of breast cancer tumour cells. This evidence concerns the gene ESR1 and breast cancer.