On the Cover: Model of the structure of Btk SH2 domain. A, A stereo view of the modeled structure of Btk SH2 domain. The analyzed XLA-causing mutations are shown on the left. On the right are amino acids predicted to be involved in the binding of pY-containing ligands. The peptide bound to the domain has sequence pYEEI. Residues recognizing and binding phosphotyrosine are colored with orange, and those binding to pY + 1 are in green. The hydrophobic pY + 3 binding pocket is in yellow. Y334, participating in both pY + 1 and pY + 3 binding, is in white. B, Binding of the pYEEI peptide to the Btk SH2 domain. Residues involved are colored as in A. The SH2 domain is shown with the surface presentation, and the peptide with a stick model. The binding residues are clearly visible. The peptide-SH2 domain interaction indicates structural complementarity. (Figure 1 from Mattsson, P. T., I. Lappalainen, C.-M. Backesjo, E. Brockmann, S. Lauren, M.Vihinen, and C. I. E. Smith. Six X-linked agammaglobulinemia-causing missense mutations in the Src homology a domain of Bruton's tyrosine kinase: phosphotyrosine-binding and circular dichroism analysis. J. Immunol. 164:4170.)