The levels of cellular messenger RNA transcripts can be regulated by controlling the rate at which the mRNA decays. Because decay rates affect the expression of specific genes, they provide a cell with flexibility in effecting rapid change. Moreover, many clinically relevant mRNAs--including several encoding cytokines, growth factors and proto-oncogenes--are regulated by differential RNA stability. But what are the sequence elements and factors that control the half-lives of mRNAs?