Colorectal carcinoma RKO cells expressing reduced levels of the RNA-binding protein HuR (ASHuR) displayed markedly reduced growth. In synchronous RKO populations, HuR was almost exclusively nuclear during early G1, increasing in the cytoplasm during late G1, S and G2. The expression and half-life of mRNAs encoding cyclins A and BI similarly increased during S and G2, then declined, indicating that mRNA stabilization contributed to their cell cycle-regulated expression. In gel-shift assays using radlolabeled cyclin RNA transcripts and RKO protein extracts, only those transcripts corresponding to the 3'-untranslated regions of cyclins A and BI formed RNA-protein complexes in a cell cycle-dependent fashion. HuR directly bound mRNAs encoding cyclins A and BI, as anti-HuR antibodies supershifted such RNA-protein complexes. Importantly, the expression and half-life of mRNAs encoding cyclins A and BI were reduced in ASHuR RKO cells. Our results indicate that HuR may play a critical role in cell proliferation, at least in part by mediating cell cycledependent stabilization of mRNAs encoding cyclins A and B1.