User Manual - File Formats - MRC/CCP4
The developers of Situs, Sculptor, Chimera, VMD, and em2em have coordinated their implementation of the CCP4-derived map formats such that maps are shown consistently across programs. The MRC format specification was originally identical to the CCP4 file format. Over time the format was changed slightly in a couple of details and now the different map formats are not necessarily fully compatible to each other. As a response to this development, the ASCII-based Situs format was developed in 1999 to provide an unambiguous representation. Today, Sculptor supports both CCP4/MRC family formats and the classic Situs format and follows the conventions described in the following document.

For more details about the conventions regarding various MRC and CCP4 formats (and Situs format) see the extensive revised documentation at the Situs web site.