Agnes Hamon,1–3 Marie Tosolini,3–6 Bernard Ycart,1–3 Frederic Pont,3–6 Jean-Jacques Fournie3–6 1Université Grenoble-Alpes, 2Laboratoire Jean Kuntzmann, Grenoble, 3Laboratoire d'Excellence, Toucan, 4INSERM UMR1037, Cancer Research Center of Toulouse, Toulouse, 5Université Toulouse III Paul-Sabatier, Toulouse, 6ERL 5294 CNRS, Toulouse, France Abstract: Cancer cells cocultured in vitro show unexpected differential growth rates that classical exponential growth models cannot account for. Two noninteracting cell lines were grown in the same culture, and counts of each species were recorded at periodic intervals. The relative growth of population ratios was found to depend on the initial proportion, in contrast with the traditional exponential growth model. A proposed explanation is the variability of growth rates for clones inside the same cell line. This leads to a log-quadratic growth model that provides both a theoretical explanation for the phenomenon that was observed, and a better fit for our growth data. Keywords: cancer cells, growth, log-linear model