Seven in absentia homolog 2 (SIAH2), a homologue of Drosophila seven in absentia (Sina), has emerged as an oncogene and plays important roles in cancer development and progression. This study further assessed the role of SIAH2 in breast cancer and the underlying molecular events. The data showed that SIAH2 protein was overexpressed in invasive breast cancer (IBC) compared to the expression noted in normal or ductal carcinoma in situ (DCIS) tissues, expression of which is associated with malignant behaviors. SIAH2 may function differently in different molecular subtypes (e.g., luminal- vs. basal-like type) of breast cancer. Manipulation of SIAH2 expression led to a 'cross-talk' of the ERK and PI3K pathway, which could be one of the mechanisms by which SIAH2 regulates viability, apoptosis, and invasion capacity in these breast cancer cell lines.