[Chimera-users] Percent Identity for specified parts of a structure

Hi there, I don't know if this command already exists, but I can't seem to find it in the Chimera tools or manual. I'd like to be able to calculate the percent identity of specified parts of 2 matched sequences - in my case the individual transmembrane helices of the structures. I wonder if a command already exists or if there is a way of doing this in Chimera without having to make new .pdb files of each of the areas of the proteins that I want to compare? I think it would be a useful command if it does not already exist. Thanks, Vicky