Our research focuses on understanding the molecular mechanisms that drive tumor progression and metastasis in breast cancer, with particular emphasis on epithelial–mesenchymal transition (EMT), cell migration, and the tumor microenvironment. We have identified key regulatory mechanisms controlling the activity of the small GTPase RAC1, including SUMOylation and ubiquitination, which modulate cell migration and EMT initiation. Our work has also demonstrated the relevance of the SUMO machinery and RAC1 SUMOylation in tumor invasion, highlighting the therapeutic potential of targeting the RAC1–SUMO pathway in metastatic breast cancer. In parallel, we investigate how EMT regulators shape the tumor microenvironment, identifying RAC1 and the transcription factor SNAI2 as key modulators of stromal–epithelial interactions that influence tumor development and malignancy. Overall, our goal is to better understand the mechanisms initiating the metastatic program in order to identify new therapeutic strategies to prevent cancer progression.