CCNA (Cyclin A:Cdc2) complexes translocate to the nucleus and may associate with condensing chromosomes in prophase (Pines and Hunter 1991). Experiments with deletion mutants of chicken CCNA2 expressed in HeLa cells have shown that the nuclear localization of cyclin A correlates on its ability to bind to CDKs (Maridor et al. 1993), which was confirmed with human CCNA2 (Jackman et al. 2002). Neither CDK1 (Kong et al. 2000), nor CDK2 (Jackman et al. 2002), nor cyclin A (Jackman et al. 2002) possess nuclear localization signals (NLSs). Cell fusion assays have demonstrated that cyclin A shuttles between the nucleus and the cytoplasm, and its nuclear localization can be explained by a more rapid nuclear import than export (Jackman et al. 2002). Nuclear import of cyclin A depends on a functional RAN GTPase, while export does not depend on exportin 1 (Jackman et al. 2002). As nuclear localization experiments used CCNA2 constructs, CCNA2 is annotated as a CCNA set member, while CCNA1 is annotated as a set candidate.