Fig. 1. Schematic of competitive release in a tumor.

– (a) Prior to treatment, a tumor consists of a large population of sensitive cells (red) and a small population of less fit resistant cells (green) competing for resources with the surrounding healthy cells (blue); (b) Chemotherapy targets the sensitive population (middle), selecting for the less fit resistant population that thrives in the absence of competition from the sensitive population; (c) Upon regrowth, the tumor composition has larger numbers of resistant cells, rendering the subsequent rounds of treatment less effective.