Hi I'm trying to join two models together using the Build Structure tool but I'm having problems. Here is my process. 1. Using the 3R2P crystal structure I select residues 3-130 from chain A and 131-182 from chain B 2. Then using the following command in Chimera I select the N atom of residue 130 that will form a bond with the C atom of residue 131: sel #0:130@N#1:131@C (#0 is the name of the 3-130 segment and #1 is the name of the 131-182 segment) 3. Then using the Build Structure tool I select Form C-N peptide bond, but the Apply button doesn't activate. I don't understand what the issue is. Is the command that I've written wrong? I would be very glad for your help. I can provide the pdb files of the residue segments if necessary. Thanks Tiglath