The relocation of PCNA from the nucleus to the
cytoplasm at this time was unexpected, although this
has been reported previously in UV-irradiated melanocytes
associated with a G2 arrest (Iyengar, 1994).
The relocation to the cytoplasm would act to reinforce
a block on S phase entry, suggesting the cells undergo
a cell cycle arrest either in G2 phase immediately after
the first round of DNA replication, or in the
subsequent G1. The elevated levels of cyclin B1,
accumulation of inactive phosphotyrosine 15 form of
cdc2 complexed with cyclin B1, a marker of G2 arrest
(Gabrielli et al., 1997b), and the lack of mitotic ®gures
(unpublished observations), all point to a cell cycle
arrest in G2 phase from 24 ± 48 h.