The lengths of 5UTRs of multicellular eukaryotes have been suggested to be subject to stochastic changes, with upstream start codons (uAUGs) as the major constraint to suppress 5UTR elongation. However, this stochastic model cannot fully explain the variations in 5UTR length. We hypothesize that the selection pressure on a combination of genomic features is also important for 5UTR evolution. The ignorance of these features may have limited the explanatory power of the stochastic model. Furthermore, different selective constraints between vertebrates and invertebrates may lead to differences in the determinants of 5UTR length, which have not been systematically analyzed.