Page "Nuclear localization sequence" Paragraph 1

Classical NLSs can be further classified as either monopartite or bipartite.

The first NLS to be discovered was the sequence PKKKRKV in the SV40 Large T-antigen ( a monopartite NLS ).

The NLS of nucleoplasmin, KRKKKK, is the prototype of the ubiquitous bipartite signal: two clusters of basic amino acids, separated by a spacer of about 10 amino acids.

Both signals are recognized by importin α. Importin α contains a bipartite NLS itself, which is specifically recognized by importin β.

The latter can be considered the actual import mediator.