Let us note here that ligand-based control of flexibility can also be seen as an "on/off" state, controllable through chemical reactions inside the MOF framework itself. J ędrzejowski et al. presented an original and inspiring example of this approach in 2021, by developing a series of 3D hybrid MOF-HOF porous materials incorporating dienophiles of increasing bulkiness, based on the JUK-20 framework (JUK = Jagiellonian University in Kraków). By using the [4 + 2] click reactions on JUK-20, they obtained a series of JUK-20-dienophile MOFs in a single-crystal-to-single-crystal transformation. These new MOFs were then shown to display varying structural flexibility, directly linked to the bulkiness of each dienophile -due to the modulation of host entropy, i.e., the number of its possible internal motions. Finally, ligand substitution can also control flexibility when it manifests in forms other than adsorption-induced structural transitions. Recently, Klokic et al. reported an interesting case of tunable behavior of photo-responsive MOF films through linker functionalization. These authors showed that introducing chlorine substituents in an azobenzene-infiltrated DMOF-1 film structure induced a significantly slow-down of the structural response upon photo-switching by UV irradiation. This orthogonal control of flexibility through both light and functionalization appears very promising for applications.