  By collecting from literature data the experimental evidences of anomalous
diffusion of passive tracers inside cytoplasm, and in particular of
subdiffusion of mRNA molecules inside live E. coli cells, we get the
probability density function of molecules' displacement and we derive the
corresponding Fokker-Planck equation. Molecules' distribution emerges to be
related to the Kr\"atzel function and its Fokker-Planck equation be a
fractional diffusion equation in the Erd\'elyi-Kober sense. The irreducibility
of the derived Fokker-Planck equation to those of other literature models is
also discussed.
