LIM-only protein 4 (LMO4) is overexpressed in over 50% of primary breast cancers and its expression is a marker of poor prognosis of the disease, but the mechanisms by which this overexpression contributes to breast cancer are not well understood. One potential mechanism is the interaction of LMO4 with the tumour suppressor CtIP. We found that in the presence of its main binding partner, LDB1, LMO4 can no longer interact with CtIP. LMO4 contains two tandem LIM domains (LIM1 and LIM2) Our solution structure of a CtIP/LMO4-LIM1 complex, demonstrated that CtIP and the LIM interaction domain of LDB1 (LDB1-LID) bind LMO4-LIM1 in an identical fashion: both partners augment an existing β-hairpin in LMO4 to form a short antiparallel β-sheet. Thus, CtIP and LDB1 are likely to compete for binding to LMO4¹. We hypothesise that overexpression of LMO4 may disrupt some of the normal tumour suppressor activities of CtIP, thereby contributing to breast cancer progression, making inhibition of this LMO4 interaction a potential target for therapeutics. Unlike LDB1, which binds all LMO and related LIM-homeodomain proteins, CtIP is believed to specifically interact with LMO4. By combining the LMO4-LIM1 interacting region of CtIP with the LMO4-LIM2 interacting region of another LMO4 binding partner, Deaf1, we constructed new chimeric LID-like sequences and assessed their relative affinities and specificities for LMO4. In a yeast-two-hybrid assay these chimeric LID peptides appear to bind better to LMO4 than does CtIP, and to a smaller subset of LIM domain-containing proteins than does LDB1. Surprisingly, denaturation assays performed on LMO4-LID fusion proteins indicated no gain in affinity relative to CtIP. We are currently investigating other methodologies for assessing binding affinity and creating other peptides with appropriate properties.