Basic rules for colonization by ENC agents (see Fig. 3). Motility rule, whereby an ENC agent has equal probability of moving a unit step in any direction (an unbiased random walk) Proliferation rule, whereby mitosis occurs in ENC agents during migration, but the proliferation rate decreases to zero as the ENC agent density increases to a preferred cell density, called the carrying capacity (this describes logistical growth). Occupancy rule, whereby at most one ENC agent can occupy a site.