Viral protein necessary for the penetration of the viral genome into the host cell nucleus either via active nuclear transport through nuclear pore complexes (NPCs) or DNA injection through the nuclear membrane. Nuclear membrane permeabilization might also be possible. Viruses can also enter the nucleus during mitosis when the nuclear membrane temporarily disintegrates (e.g. most retroviruses). All these strategies to cross the nuclear envelope barrier are associated with various levels of capsid disassembly, since virus can pass intact (e.g. papovaviruses) or, in the case of injection, only the viral genome enters the nucleus (e.g. herpesviruses).