CTS units, meaning criminal test subject unit, are expendable test subjects used by Nero when researching entities in containment. CTS units are drawn worldwide from the ranks of prison inmates convicted of violent crimes, especially those on death row. During periods of low prisoner supply, Nero is allowed to recruit from other sources, such as political prisoners, refugee populations, and other civilian sources.

After retrieval, CTS units are brought to a facility chosen beforehand, where they are shown the "CTS Training Video" (updated last in 2014) and shown their quarters.

CTS units are given a code instead of a name, following a template such as, "C-[five numbers]-[two-letter abbreviation of country of origin]". While most CTS units keep their original name, Nero staff are told to only call them by their given code.

Since CTS units are expendable, codes can be recycled, except in cases where a CTS unit was a vital part in an important discovery. However, these situations are rare.