A small difference in intermolecular binding affinity may change organismal phenotype. (a) The interaction of three basic residues of a Drosophila Hox protein, Sex combs reduced (Scr), with the minor groove of fkh250 DNA. (b) KD values of wild-type Scr and three alanine mutants binding fkh250. (c) Drosophila embryos ubiquitously expressing ScrWT and Scr mutants. Arrowheads indicate formation of salivary glands. Reprinted from Cell, volume 131, R Joshi, JM Passner, R Rohs, R Jain, A Sosinsky, MA Crickmore, V Jacob, AK Aggarwal, B Honig, and RS Mann, Functional specificity of a Hox protein mediated by the recognition of minor groove structure, pages 530-543, copyright (2007), with permission from Elsevier.