This is the file format used by many popular programs like GARLI, GDA, MacClade, Mesquite, ModelTest, MrBayes and PAUP*. Nexus file names often have a .nxs or .nex extension.
A formal description of the NEXUS format can be found in Maddison et al. (1997).
Conversion of an interleaved NEXUS file to a non-interleaved NEXUS file: execute the file in PAUP*, and export the file as non-interleaved NEXUS file. You can also type the commands:
export file=yourfile.nex format=nexus interleaved=no;