A novel peptide (JBS2) designed against the cis-dimerization site [25] of JAM-A was confirmed to bind to recombinant human JAM-A and to lower the viability of ER-negative HER2-positive SUM-225 cells, HER2-negative MCF7 cells, and a patient DCIS primary culture in vitro. This evidence concerns the gene ERBB2 and ductal breast carcinoma in situ.