The ReactOS kernel is their own, clean room reverse engineered to be compatible with the NT kernel. Also just a side note, Linux isn't exactly a UNIX fork eith

The ReactOS kernel is their own, clean room reverse engineered to be compatible with the NT kernel. Also just a side note, Linux isn't exactly a UNIX fork either. Linux was developed from scratch on a MINIX host, which in turn was another OS designed to be UNIX-compatible but did not actually use any UNIX code. To put it another way, it's Unix-like two generations removed, but definitely not something you could call a fork. That might seem like a small detail to you but SCO vs IBM says otherwise.