Cell culture competition. Image of competition between Barrett's esophagus cell line CP-D (green) and normal squamous cell line EPC2 (red). Cells are grown until they begin to approach confluency and are then passaged to maintain logarithmic growth. Both cell lines are labeled here for imaging purposes, however, in standard competitions BE cell lines are unlabeled. Note that the green cells do not necessarily grow in contiguous patches, and so there must either be cell migration or a process of detachment and reattachment as those clones expand. The two cell types can interact both by physical contact and via secreted factors.