AMX Mod X was originally based on AMX Mod, a similar Half-Life modification by OLO. However, AMX Mod began to slip into poor project policies (lack of public input, no open source enforcement, no central source code maintained, and much more).
Due to the dead appearance of the project as all maintainers had disappeared into oblivion, the AMX Mod X project was started.
Since then, the AMX Mod X project has come a long way. Nearly every portion of AMX Mod has been either rewritten from the ground up, cleaned up, or changed to work in a new system.
AMX Mod X is not AMX Mod. And better yet, it`s almost fully backward compatible.