  Biology offers many examples of large-scale, complex, concurrent systems:
many processes take place in parallel, compete on resources and influence each
other's behavior. The scalable modeling of biological systems continues to be a
very active field of research. In this paper we introduce a new approach based
on Event-B, a state-based formal method with refinement as its central
ingredient, allowing us to check for model consistency step-by-step in an
automated way. Our approach based on functions leads to an elegant and concise
modeling method. We demonstrate this approach by constructing what is, to our
knowledge, the largest ever built Event-B model, describing the ErbB signaling
pathway, a key evolutionary pathway with a significant role in development and
in many types of cancer. The Event-B model for the ErbB pathway describes 1320
molecular reactions through 242 events.
