FK-506 is a potent immunosuppressant that forms a high affinity complex (Ki = 0.2 nM) with FK-506 binding protein 12 (FKBP12). This complex inhibits the activity of the calcium/calmodulin-dependent protein phosphatase, calcineurin, leading to disruption of T-cell activation.{13842} SLF is a cell-permeable analog of FK-506 that binds tightly to FKBP but lacks the ability to inhibit calcineurin.{13706} By itself, SLF does not exhibit a significant cellular response. However, homodimers of SLF or heterodimers with another protein ligand, provide a method to promote the dimerization of ligand-specific fusion proteins to induce signaling events within a cellular environment.{13701,13706} For example, SLF monomers connected with a 10-atom linker activates Fas signaling via Fas-FKBP fusion proteins with an EC50 of 6 nM.{13701}