It is reported that patients with Huntington's disease (HD) have a low incidence of cancer. In this study, we investigated the expression of huntingtin-associated protein 1 (HAP1), the ligand of HD's production, in breast tumor and normal tissues. We found that HAP1 expression was significantly lower in tumor compared to normal tissues. We then transfected the HAP1 gene into the breast cancer lines MCF-7 and MDA-MB-231, and results showed that the overexpression of HAP1 reduced the growth of the two cell lines. In addition, we observed that HAP1 also reduced invasion and migration, and upregulated apoptosis in MCF-7 cells; however, these changes were not observed in MDA-MB-231 cells. We also demonstrated that the expression of EGFR and apoptosis-related genes might be involved in cell proliferation and apoptosis. In conclusion, overexpression of HAP1 reduced in vitro cell growth in breast cancer cell lines, suppressed the migration and invasion, and promoted the apoptosis of certain cell lines. Therefore, HAP1 is a potential molecular target for the diagnosis and treatment of breast cancer.