CD44 and renal cell carcinoma: To test this hypothesis, we evaluated the expression of RSK4, CD44 and MMP-9 in mRCC tissues, analysed the relationship between RSK4, CD44 and MMP-9 expression and the clinicopathological features of mRCC patients, and explored the potential molecular mechanisms of the RSK4, CD44 and MMP-9 pathways in RCC cell lines.