FKBP4 and psychiatric disorder: As a first proof of concept, we tested binding of 1 to the FK506‐binding proteins FKBP12, FKBP51 and FKBP52,17 which have raised substantial interest as potential targets for neurological and psychiatric diseases (Figure 2).18 Indeed, 1 bound to FKBP12 with a Ki of 5±1 μm, making it one of the smallest FKBP12 ligands known.191 did not bind to the larger FKBP51 or FKBP52 up to the solubility limits.