  We develop a microscopic theory for reaction-difusion (R-D) processes based
on a generalization of Einstein's master equation with a reactive term and we
show how the mean field formulation leads to a generalized R-D equation with
non-classical solutions. For the $n$-th order annihilation reaction
$A+A+A+...+A\rightarrow 0$, we obtain a nonlinear reaction-diffusion equation
for which we discuss scaling and non-scaling formulations. We find steady
states with either solutions exhibiting long range power law behavior (for
$n>\alpha$) showing the relative dominance of sub-diffusion over reaction
effects in constrained systems, or conversely solutions (for $n<\alpha<n+1$)
with finite support of the concentration distribution describing situations
where diffusion is slow and extinction is fast. Theoretical results are
compared with experimental data for morphogen gradient formation.

Statistical Mechanics