CBFbeta, the essential co-regulator of RUNX transcription factors, has a pro-metastatic role in triple negative breast cancer cells. In this context, RUNX1-CBFbeta and RUNX2-CBFbeta activate the expression of the EMT master regulator SNAI2, thereby maintaining the mesenchymal state of the cells. However, CBFbeta is reported to be one of the most frequently mutated genes in ER positive (ER+) breast cancer. Many of these mutations are nonsense mutations and are predicted to result in loss of function, suggesting a tumour suppressor role for CBFbeta. However, the impact of missense mutations and the loss of CBFbeta in ER+ breast cancer cells has not previously been studied. Work described in this thesis demonstrates that missense mutations in CBFbeta accumulate near the Runt-domain binding region. Missense mutations inhibited the ability of CBFbeta to form CBFbeta-RUNX-DNA complexes suggesting, that these mutations are indeed loss of function. The loss of CBFbeta in ER+ breast cancer cells was then modelled by using CRISPR-Cas9-mediated gene-editing to delete CBFbeta in ER+ MCF7 cells. This led to an increase in cell migration that was dependent on the presence of ERÎ±. Analysis of the potential mechanism revealed that the increase in migration is driven by the co-regulation of Trefoil factor 1 (TFF1) by CBFbeta and ERÎ±. Subsequent analysis demonstrated that RUNX1-CBFbeta acts to repress ERÎ± activated expression of TFF1. TFF1 is a motogen that stimulates migration and it was shown that knockdown of TFF1 in CBFbeta-/- cells inhibits the migratory phenotype. The findings in this thesis therefore reveal a new mechanism by which RUNX1-CBFbeta and ERÎ± combine to regulate gene expression and a new role for RUNX1-CBFbeta in the prevention of cell migration by supressing the expression of the motogen TFF1.