siRNAs were transfected in four different cell lines: two ovarian cancer cell lines (SKOV3 [HER2 amplified; CDKN2A, PIK3CA, and TP53 mutants] and OVCAR3 [PIK3R1 and TP53 mutants]) and two breast cancer cell lines (MDA-MB-231 [BRAF, CDKN2A, KRAS, and TP53 mutants] and MDA-MB-468 [EGFR amplified; PTEN, RB1, TP53, and SMAD4 mutants]) with different genetic backgrounds. This evidence concerns the gene SMAD4 and breast cancer.