Expression of many proto-oncogenes, cytokines and lymphokines is regulated by targeting their messenger RNAs for rapid degradation. Essential signals for this control are AU-rich elements (AREs) in the 3′ untranslated region (UTR) of these messages. The ARE is loosely defined as the five-nucleotide sequence AUUUA embedded in a uracil-rich region. A trans-acting factor, presumably a protein, binds the ARE and initiates recognition by the destabilization machinery. Numerous candidate ARE-binding proteins have been proposed. We show that a 32 kDa protein in HeLa nuclear extracts characterized previously has RNA-binding specificity that correlates with the activity of an ARE in directing mRNA decay. Purification and subsequent analyses demonstrate that this 32 kDa protein is identical to a recently identified member of the Elav-like gene family (ELG) called HuR. The in vitro binding selectivity of HuR is indicative of an ARE sequence's ability to destabilize a mRNA in vivo, suggesting a critical role for HuR in the regulation of mRNA degradation.