Hi, I have a question about marker and marker link syntax. Suppose model #1 has chains A and B, and that chains A and B form a homodimer where each subunit has the same residue numbering. I would like to place a marker at the midpoint between alpha-carbons of position 10 on A and 10 on B. In turn, I'd like another marker at the midpoint between position 20 on each of the two subunits. I would then like to draw a linker between the two markers. I would expect the following syntax to work, but I get an error. marker #2 #1/a,b:10@CA color yellow radius 1 marker #2 #1/a,b:20@CA color yellow radius 1 marker link #2:1,2 color yellow radius 0.5 Gives the error: Missing or invalid "position" argument: Expected 3 floats or object specifier Any suggestions on how to fix the syntax, or is there an alternative approach? Thanks, Joel