The overall aim of the study is to provide a basis from which to improve the ability of tamoxifen family drugs to reduce the activity of a secondary target protein, calmodulin, during tumour development. The main conclusion is that the binding of a tamoxifen analogue is quite unlike that of other anti-calmodulin compounds in that two drug molecules bring the two domains of calmodulin into close proximity, but they are not fixed in orientation relative to the protein.