Abstract Background: Metastasis and therapeutic resistance are major issues in treating patients with breast cancer. Breast tumors that secrete mucus are generally resistant to chemotherapy. One type of secreted mucin, Mucin-2 (Muc2), is expressed in some breast cancers, but is absent in non-neoplastic breast tissue, suggesting that it plays a role in modulating breast cancer biology. Whether Muc2 expression correlates with breast cancer patient survival is controversial. Thus, the effects of Muc2 on the biology of breast cancer are largely unknown. This study examined the role of Muc2 in modulating breast cancer cell proliferation in vitro and in vivo, response to chemotherapy, and metastasis. Methods: Two novel cell lines were developed from patient derived tumor xenografts. These two cell lines, BCK4 and PT12, both express high levels of Muc2. In order to modulate Muc2 levels in both cell lines, shRNA targeted to Muc2 (shMuc2) were compared to non-targeting control shRNA (shNT). Decreased expression of Muc2 was confirmed using immunoblotting. Proliferation in vitro was measured using the IncuCyte live cell imaging system and crystal violet staining. Response to chemotherapy was measured by examining apoptosis using cleaved-caspase 3 expression. BCK4 and PT12 cells with shNT or shMuc2 were grown as solid tumors in immunocompromised mice and tumor volume measured by caliper. BCK4 cells with high Muc2 (shNT) and low Muc2 (shMuc2) were labeled with luciferase and examined in an experimental metastasis model where total disease was monitored by IVIS imaging. Results: Decreased Muc2 expression decreased proliferation in BCK4 and PT12 cells versus non-targeting control cells both in vitro and in vivo. Treatment with the chemotherapeutic Docetaxel induced minimal apoptosis in BCK4 control cells with high Muc2 however, apoptosis was significantly increased in BCK4 cells with reduced Muc2. In an experimental metastasis model, mice injected with BCK4 cells containing low Muc2 had decreased disease burden versus those injected with control cells with high Muc2. Endogenous Muc2 expression in wild-type BCK4 cells increased with addition of EGF and this effect was abolished by addition of the EGF-receptor inhibitor, Erlotinib. Conclusions: Muc2 expression plays an important role in mediating proliferation, apoptosis and metastasis of breast cancer cells. These data suggest that Muc2 is important in controlling the biology of Muc2 positive breast tumors. In addition, Muc2 may be important in guiding treatment and predicting outcomes in breast cancer patients. Citation Format: Astashchanka A, Shroka T, Jacobsen BM. Mucin-2 (Muc-2) modulates the biology of breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-01-13.