We investigate clustering of malignant glioma cells. \emph{In vitro}
experiments in collagen gels identified a cell line that formed clusters in a
region of low cell density, whereas a very similar cell line (which lacks an
important mutation) did not cluster significantly. We hypothesize that the
mutation affects the strength of cell-cell adhesion. We investigate this effect
in a new experiment, which follows the clustering dynamics of glioma cells on a
surface. We interpret our results in terms of a stochastic model and identify
two mechanisms of clustering. First, there is a critical value of the strength
of adhesion; above the threshold, large clusters grow from a homogeneous
suspension of cells; below it, the system remains homogeneous, similarly to the
ordinary phase separation. Second, when cells form a cluster, we have evidence
that they increase their proliferation rate. We have successfully reproduced
the experimental findings and found that both mechanisms are crucial for
cluster formation and growth.Comment: 6 pages, 6 figure