HIV can infect non-dividing cells, implying that the PIC must be able to traverse the nuclear membrane. In contrast, simple retroviruses such as MLV can only infect cells once they have passed through mitosis, potentially because they require breakdown of the nucleus to access chromosomal integration sites. The mechanism of nuclear localization is controversial. A variety of proposals have been made for nuclear localization sequences (NLS) in the PIC, but most of those have now been shown to be dispensible for HIV integration. According to a new idea from Yamashita and Emerman, it may be that the PIC is imported into the nucleus by a default pathway, while MLV PICs are retained in the cytoplasm because capsid protein is stably associated with PICs.