Several messenger RNAs which are transiently expressed contain a conserved uridine-adenosine-rich sequence in their 3' untranslated region. Many of these mRNAs encode cytokines, growth factors or oncoproteins. This UA-rich sequence is composed of several interspersed repeats of the octanucleotide UUAUUUAU and plays a key role in the post-transcriptional regulation of these mRNAs. Known as instability determinants, these UA-rich elements can also strongly affect mRNA translational regulation and give insight into the underlying mechanism.