I'm not sure where in the MRC binary file this "EXPDATA EM" is suppose to go, maybe a "label" field. As you can see from the MRC file spec it has space for 10 labels in bytes 225-1024 of the header

ChimeraX has no options to insert specified text into those labels, but you could edit the ChimeraX Python code that writes the MRC file to add a label. The file is called writemrc.py and is in your ChimeraX distribution, on Mac it is here

ChimeraX.app/Contents/lib/python3.11/site-packages/chimerax/map_data/mrc/writemrc.py

Here's what this code looks like in the ChimeraX Github repository

https://github.com/RBVI/ChimeraX/blob/00c7ef34fdf8e3f9675cf892f346adbecbfe6c44/src/bundles/map_data/src/mrc/writemrc.py#L132

It has some lines that put a ChimeraX version and time stamp into the MRC file

ver_stamp = 'ChimeraX %s %s' % (version, asctime())

labels = [ver_stamp[:80]]

You could add a line after that

labels.append('EXPDATA EM') to add a label with that string.

then restart ChimeraX and save a map (e.g. command "save mymap.mrc").

Another approach would be to open the mrc file in a binary editor and carefully modify some of the bytes from 225-1024 to add your label. Of course all this assumes that EXPDATA EM is supposed to go in a label and not for instance the EXTRA bytes part of the header. It seems absurd that you have software that won't work without a special label in the file.

Tom

On Dec 3, 2024, at 4:47 PM, Raman, C.S. <craman@rx.umaryland.edu> wrote:

Hi Tom,

Is there an easy way to insert into the header of an MRC file the descriptor "EXPDATA EM" as done by the PDB? I need to do this to run a validation program but don't quite know how to go about it.

Any help would be greatly appreciated.
-raman