Reconsidering the significance of genomic word frequency
  We propose that the distribution of DNA words in genomic sequences can be primarily characterized by a double Pareto-lognormal distribution, which explains lognormal and power-law features found across all known genomes. Such a distribution may be the result of completely random sequence evolution by duplication processes. The parametrization of genomic word frequencies allows for an assessment of significance for frequent or rare sequence motifs. 
Mikl\'os Cs\H{u}r\"os, Laurent No\'e and Gregory Kucherov