Temperature sensitive alleles of the GTP binding protein RAN, which is the S. pombe homolog of RCC1 in mammals are unable to form a normal interphase nucleus following mitosis. The cytoplasm appears to progress in interphase, but the nucleus continues to show condensed chromatin and the cells ultimately die. Electron microscopy and EM immunocytochemistry have been used to analyze mutant cells at permissive and restrictive temperatures. The work has demonstrated that the failure is due to a breakup of a nuclear envelope following mitosis at restrictive temperature. Chromatin becomes exposed to cytoplasm, and fails to perform its normal functions. The work suggests that the RAN GTPases are involved in control of nuclear structure. This work has been published in the PNAS (Demeter, et al., 1994).