Level 5 was a prison facility inside Primatech Research designed and built by the Company to contain more dangerous individuals, including evolved humans. Many of the cells were specially programmed to neutralize their captives' abilities. Level 5 was destroyed during the Primatech explosion. Level 5 also has it own power grid seperate from the rest of the site.
There are other level cells in different sections of Primatech Research as Hiro and Ando were kept in Level 2. Adam Monroe who nearly brought down the human race by releasing Shanti Virus was kept in a cell that looked like level 2.