What version of Natural is thread-safe under CICS?

Can anybody tell me at what level of CICS and Natural the environment is thread-safe?
Apologies if this has been answered before.
I searched the archives and found no matches.

NCI 4.2 is threadsafe.
But you should be aware, that all modules CALLed by Natural via standard linkage serve serve as a logical extension to Natural and have to be threadsafe too; this includes user written CSTATIC and RCA programs and of course Adabas.