Gap junction β‑2 gene (GJB2, also known as connexin 26) is a member of the connexin family which forms gap junction channels. Many connexin genes have been considered to be tumor suppressor genes. However, the overexpression of GJB2 has been found to be associated with a poor prognosis in several human cancers. In our previous microarray study, we revealed the overexpression of GJB2 in breast cancer tissues. Hence, in this study, we investigated the expression of GJB2 in human breast cancer and its role in breast cancer cell proliferation and migration. The RT‑qPCR results revealed the upregulation of the GJB2 gene in invasive ductal carcinoma (P<0.001) of the breast. Immunohistochemical analysis revealed an intense cytoplasmic and membrane staining. We observed that the staining for GJB2 was more intense in the majority of the estrogen receptor (ER)‑negative breast cancer tissues compared to the normal breast tissues (P<0.0001). By contrast, the majority of the ER‑positive breast cancer samples exhibited weak to moderate staining; however, this difference was not statistically significant compared to the normal tisues. The knockdown of GJB2 in human breast cancer cell lines using shRNA led to a significant decrease in the proliferative ability and an increase in the migratory ability of breast cancer cells. In addition, the knockdown of GJB‑2 led to a significant reduction in tumor volume and proliferation (as demonstrated by MIB‑1 staining) in orthotopic xenografts in immunocompromised mice. On the whole, the findings of this study indicate that GJB2 may be an important regulator of breast tumorigenesis.