To conclude, the hypermethylation of DLG3 and lowly expressed DLG3 are revealed in BC cells and tissues in vitro and in vivo; furthermore, the in-depth mechanism study confirms that the decrease of DLG3 expression level contributes to the upregulation of RAC1 and the abnormal activation of the PI3K/AKT pathway, thus affecting BC cell invasion, metastasis, and EMT. Here, RAC1 is linked to breast cancer.