LIM-homeodomain (LIM-HD), LIM-only (LMO) proteins and their co-factors act within a transcriptional code that regulates cell specification in the central nervous system. The LIM domains in these proteins bind short intrinsically disordered regions (IDRs) in their partner proteins which undergo disorder-to-order transitions upon binding. The establishment of cell specific transcriptional complexes involves multiple competing IDR-LIM protein interactions. It was difficult to evaluate these interactions previously, as the LIM domains are obligate binders that aggregate in the absence of a binding partner. We have developed Förster resonance energy transfer (FRET)-based experiments for measuring the binding affinity, off-rates and inferred on-rates for this network of protein-protein interactions [1]. Our data reveal that the interactions made by these closely related proteins can have remarkably different properties in terms of affinities, kinetics of binding and mechanisms of binding, despite forming very similar structures. Modelling the population distributions of complexes based on these data reveals an important role for binding kinetics on transcriptional complex formation, where weak binding affinities teamed with surprisingly slow off rates predominate over sets of stronger, faster interactions to drive the expression of target genes and competition for binding partners inhibits the formation of inappropriate complexes for a given cell type.