FSM was developed by Ming Guo and David A. Weitz to probe stochastic intracellular forces generated by motor proteins. Far from a liquid void, the cytoplasm contains a complex meshwork of actin and myosin conferring structural support to the cell, as well as harbouring vesicles and mitochondria among other organelles. Recent research on the macromolecular crowding inside the cytoplasm raises concerns whether diffusive-like motion of large molecules have been mistakenly attributed to Brownian forces. Instead, there are suspicions that myosin motor proteins, which tug randomly on the actin filaments embedded with large molecules, give rise to diffusive-like motion of molecules inside cells. Guo et al. developed an assay to distinguish whether particle motion inside cells are driven by thermal diffusion or by repercussions from active motor proteins like non-muscle myosin II shaking the cellular cytoskeleton.
