We have cloned, expressed, purified and crystallised the first DH GEF domain of Kalirin (gene: KALRN) in complex with RAC1 (gene: RAC1) as part of a programme to explore a new way of targeting GTPases. Fragment screening and X-ray crystallography has identified binders within the orthosteric binding site. A nucleotide exchange assay has been developed and GEF activity established.