Human cancers arise from defects in regulatory pathways that control the normal proliferation and differentiation of cells. The predominant cause of cancer mortality (~90%) is metastatic tumours inducing pathology in vital organs. Pivotal to carcinogenesis is deregulation of the cell cycle machinery leading to tumour growth at the primary and metastatic sites. Cell division is mediated by the cyclin-dependent kinases (CDKs), which phosphorylate substrate proteins to promote cell cycle progression and it is clear that deregulated CDK activity is important in many human cancers. A critical CDK substrate is the retinoblastoma tumour suppressor protein, pRb, which inhibits cell cycle progression. Recently published studies from our laboratory have shown that CDK-mediated phosphorylation pRb and its binding partner, termed the retinoblastoma binding protein 1 (RBP1) is important for overcoming the cell cycle inhibitory functions of these proteins. In addition to binding pRb, RBP1 binds to the breast cancer metastasis suppressor protein 1, BRMS1. BRMS1 suppresses metastasis of various cancer cells. Our data now show that BRMS1 is also phosphorylated by CDKs and this reduces the ability of this protein to inhibit the migration of breast cancer cells. These data for the first time suggests that deregulated CDKs may promote two fundamental aspects of carcinogenesis, proliferation and metastasis.