Summary: Finally, the mode of migration of invading fibro-sarcoma cells inside 3D collagen-I gel was studied. Using reflection-confocal imaging, the deformation of collagen gel as a result of forces applied by a migrating cell was analyzed and it was found that at the leading edge of the cell, the displacement of the gel and that of the cell follow a cyclic correlation and this cyclic behavior aids migration. A simple model was proposed to explain the stages of cell-migration in 3D matrices. We propose that applying the same analysis to cells wherein key focal-adhesion proteins are absent or inactive, shall elucidate the role of these proteins in 3D motility of cancer cells.