Hi Bernard, Thanks for the info on the history of signed versus unsigned 8-bit MRC files. I see that CCP4 clearly uses signed 8-bit and most MRC used for EM data uses unsigned. Is there any definitive specification for MRC format? I just made up that bit about an "MRC standard". I'm not aware of anyone publishing a standard. The Chimera change was motivate by the EM Databank using CCP4 for their maps. A possible more explicit handling as you suggest is to assume signed 8-bit for file suffixes ".ccp4" and ".map" (used by EMDB) and unsigned 8-bit for the file suffix ".mrc" -- basically distinguish CCP4 from MRC format in Chimera. I'm not sure if the ".map" suffix used by EMDB to mean CCP4 is in use by other EM labs to mean MRC. Tom