  We study the length distribution of a particular class of DNA sequences known
as 5'UTR exons. These exons belong to the messanger RNA of protein coding
genes, but they are not coding (they are located upstream of the coding portion
of the mRNA) and are thus less constrained from an evolutionary point of view.
We show that both in mouse and in human these exons show a very clean power law
decay in their length distribution and suggest a simple evolutionary model
which may explain this finding. We conjecture that this power law behaviour
could indeed be a general feature of higher eukaryotes.
