The extracellular signal-regulated protein kinase 5 (ERK5), also known as big mitogen-activated protein (MAP) kinase 1 (BMK1), is a non-redundant mitogen-activated protein kinase (MAPK) implicated in mediating the response of cells to mitogens, oxidative and osmotic stresses. The molecular complexity of the ERK5 cascade has been mostly delineated by over-expression studies. For example, like other MAPKs, ERK5 activity increases upon phosphorylation by a MAPK/ERK kinase, namely MEK5. However, the physiological role of ERK5 is not rigorously established by these data. Furthermore, in comparison to the other members of the family, little is known about the downstream targets of ERK5. This constitutes an obstacle for the molecular understanding of the signalling mechanisms that account for the effect of ERK5 activation in vivo. To clarify these issues, I have tested the effect of the conditional loss of ERK5 in primary mouse embryonic fibroblasts (MEFs). My results indicate that ERK5 is required for the proliferation of MEFs, at least in part, by promoting the entry into S phase of the cell cycle. ERK5 suppressed the expression of the cyclin-dependent protein kinase (CDK) inhibitors, p21 and p27. As a result, low-level CDK2 activity detected in ERK5-deficient MEFs correlated with hypo-phosphorylation of the retinoblastoma (Rb) protein and with a defect in G1 to S phase transition of the cell cycle. ERK5 blocks p21 expression by decreasing the stability of the p21 transcript. This process might, at least partially, involve a mechanism implicating c-Myc-induced transcriptional up-regulation of the miR-17-92 cluster. Concerning p27, ERK5 decreases p27 protein stability. The stabilisation of p27 in the absence of ERK5 resulted in the accumulation of the protein in the nucleus. To examine the relevance of my findings in cancer, I tested the effect of pharmacological inhibition of ERK5 in two human breast cancer cell lines, MCF7 and MDA- MB-231, using XMD8-92, a novel potent and selective inhibitor of ERK5. My results show that these cells are dependent on ERK5 to proliferate. Furthermore, I found that incubation of MDA- MB-231 cells with XMD8-92 compromised their ability to invade. In both breast cancer cell lines, ERK5 down-regulates p21 and p27 expression. Together with evidence that cancer patients with poor prognosis display a high-level of expression of components of the ERK5 signalling pathway, these findings support the hypothesis that ERK5 can be a potential target for cancer therapy.