The CAL-51 cell line serves as a vital in vitro model for aggressive breast cancer research, particularly in the realm of triple-negative breast cancer (TNBC). Derived from a pleural effusion metastasis, this human cell line demonstrates unique properties such as adherent growth, an epithelial-like morphology, and a normal diploid karyotype. CAL-51 lacks the expression of estrogen receptors, progesterone receptors, and HER2, positioning it as an invaluable tool for studying various aspects of TNBC, including tumorigenicity and therapeutic resistance. Its documented use in preclinical studies, including those involving SARS-CoV-2, further enhances its utility for research and drug development.