  The MAP kinase cascade is a network of enzymatic reactions arranged in
layers. In each layer occurs a multiple futile cycle of phosphorylations. The
fully phosphorylated substrate then serves as an enzyme for the layer below.
This papers focusses on the existence of parameters for which Hopf bifurcations
occur and generate periodic orbits. Furthermore it is explained how geometric
singular perturbation theory allows to generalize results from simple models to
more complex ones.
