Selection of phage-displayed peptides binding to rituximab. (A) Phages displaying the enriched peptides specifically bind to rituximab but not to the control isotype antibody basiliximab. Phages (5 × 10⁷ transducing units [TU]) displaying no peptide insert (fd-tet) or the peptides enriched by selection on rituximab were incubated on immobilized rituximab or the basiliximab control, respectively. Bound phages were recovered by K91 bacterial infection. Transduced bacteria were grown on LB plates containing tetracycline to determine the number of TUs by colony counting. Data are means from triplicate platings ± SEM. As binding is approximately 80 000 times stronger to rituximab compared with basiliximab, this background binding level is too low to be visible in the graph. (B) “Translated” insert sequences display sequence homology to the CD20 protein. The aromatic amino acids and the threonine within the enriched peptide sequences were replaced with tyrosine and serine, respectively. These translated sequences revealed sequence homology to the YCYSI string within the putative extracellular domain of CD20. aa indicates amino acids.