Before revision:  In {\em Wang and Sontag, 2008 ,the authors study the number of steady states in general  n-site sequential distributive phosphorylation and show that there are at most 2n-1 steady states.
After revision:  Many proteins are phosphorylated in a sequential and distributive way at more than one phosphorylation site. Mathematical models of n-site sequential distributive phosphorylation are therefore studied frequently. In particular, in {\em Wang and Sontag, 2008 ,the authors study the number of steady states in general n-site sequential distributive phosphorylation and show that there are at most 2n-1 steady states.