Experimental findings. Among the recent experimental findings that help us better understand the influence of crystal size on the flexibility of framework materials, we choose to highlight here a few studies of importance. The influence of crystal size on the flexibility of mesoporous MOF DUT-49 was originally established in 2018: Krause et al. showed that the adsorption-induced structural transition was diminished for samples with small crystal sizes, with the flexibility even being completely suppressed for some guest molecules. While this effect was clearly demonstrated, it was not fully understood at the time. In a later study, the same group successfully used in situ 129 Xe NMR spectroscopy in order to probe the influence of defects and crystal size on the flexibility of mesoporous MOF DUT-49. Using an analysis of the chemical shift at different pressures as a proxy for measuring the solid-fluid interaction strength, they showed that in smaller crystals, the host-guest interaction in the saturated pores are strongly decreased, minimizing the driving force for structural transitions, ascribing a microscopic mechanism to this macroscopic phenomenon.