The functioning of a living cell is largely determined by the structure of its regulatory network, comprising complex, typically not pairwise interactions between regulatory genes. An important factor for the stability and evolvability of such regulatory systems is neutrality --- typically a large number of alternative structures exhibit the same dynamical behaviour.
Here we study the regulatory network for the cell cycle of budding yeast (Saccharomyces cerevisiae). A coarse-grained Boolean approach (Li et al, PNAS, 2004) allows us to abstract from biochemical details such as precise binding constants and degradation rates. We use a sampling algorithm of Wang-Landau type to estimate the number of different yeast networks fulfilling the task of cell division. We also study the reachability of the alternative networks from the wild type. This neutral space of regulatory networks is fragmented under point mutations which establish or delete single interactions.