background: Triple-negative (TN) breast cancer represents a subtype of breast cancer that does not express estrogen receptor (ER), progesterone receptor (PR), or human epidermal growth factor receptor 2 (HER-2). Clinically, it is characterized by high invasiveness, high metastatic potential, and poor prognosis. Inhibitor of DNA binding 4 (ID4) has been shown to be overexpressed in these tumors acting as an oncogene responsible for many of its aggressive features. CDC42, a plasma membrane-associated small GTPase, can downregulate ID4 gene expression through hypermethylation of its promoter in colorectal adenocarcinomas. Since ID4 acts as an oncogene and is hypomethylated in TN breast tumors, here we asked whether CDC42 could also epigenetically silence ID4 and in doing so revert aggressive features of this tumor type. methods: Gene expression was retrieved from TCGA database using UCSC Xena. Association between overall survival (OS) and gene expression was assessed using Kaplan-Meier plotter. In vitro experiments involved ectopic expression of CDC42 in MDA-MB231and in MDA-MB468 breast cancer cell lines. Gene expression was analyzed by qPCR, western blot and inmunofluorescence assays and methylation by MSP, MS-MLPA, or ddMSP. results: Data mining analysis revealed that CDC42 expression varies among breast cancer subtypes that in the basal-like subtype there is an inverse correlation between CDC42 and ID4 expression and a positive correlation between CDC42 expression and ID4 methylation. In vitro experiments revealed that CDC42 overexpression induced ID4 methylation through the activation of the EZH2 pathway. ID4 silencing produced an increase in BRCA1 expression and a less aggressive phenotype in the tested cell line. conclusions: We show that CDC42 silences ID4 through methylation in TN breast cancer. Given that ID4 acts as an oncogene in these tumors, we think that finding an epigenetic regulator of ID4 contributes to the research and clinical management of TN breast tumors.