The Suppressor Of Cytokine Signalling (SOCS) family of proteins are critical negative regulators of cytokine and growth factor signalling. Whilst the roles of some SOCS family members have been clearly elucidated, the function of SOCS5 remains poorly characterised. SOCS5 has been postulated to act as a tumour suppressor in both EGFR and JAK/STAT-driven malignancies through expression analyses and functional studies of the Drosophila homologue Socs36E. However its precise physiological role and how it acts on these distinct pathways is yet to be elucidated. SOCS5 contains a C-terminal SOCS box motif that forms part of an E3 ubiquitin-ligase complex, a central SH2-domain and large uncharacterised N-terminal region. In this study, we have used mass spectrometry to characterise the SOCS5 interactome and have identified a number of known and novel interacting proteins. These analyses have also revealed that the SOCS5 N-terminal region, which is predicted to be largely unstructured, to be heavily phosphorylated and we hypothesis that SOCS5 acts as a scaffold to support multiple signalling complexes. Additionally, many of the proteins enriched by SOCS5 are known drivers of tumorigenesis, particularly in the context of breast cancer. Expression analysis of SOCS5 in breast cancer revealed that is down regulated in a majority of patients with invasive ductal breast carcinoma (TCGA dataset, top 7% under-expressed genes, p=2.53E-23). Excitingly, SOCS5-deficient mice exhibited accelerated tumour onset and growth relative to wild-type mice in the Polyoma Middle T antigen model of breast cancer, thus providing the first in vivo evidence that mammalian SOCS5 can act to regulate tumorigenesis. Using our interactome data, we are now seeking to determine which specific protein targets are regulated by SOCS5 in breast cancer cells.