Fig. 4. Model of epigenetic memory at PRE/TREs during the cell cycle. A model based on published findings (Buchenau et al., 1998; Schmitt et al., 2005; Sanchez-Elsner et al., 2006). (1) During interphase, PRE/TREs silence by default. Only those PRE/TREs that are transcribed escape this silencing. (2,3) After replication, transcription through the PRE/TRE continues to counteract silencing. (4) At the onset of mitosis, the PcG proteins PSC, PH and PC (Posterior sex combs, Polyhomeotic and Polycomb, respectively) dissociate simultaneously from chromatin (Buchenau et al., 1998). (5) During mitosis, there is a global shutdown of transcription. Those PRE/TREs that were transcribed in the previous interphase must somehow be marked. (6) The PcG proteins reassociate with chromatin at different points during late mitosis. PSC returns during anaphase, PH in telophase and PC at the beginning of interphase (Buchenau et al., 1998). The transcription of marked PRE/TREs must resume before a functional PcG complex has assembled at PRE/TREs. This would prevent default silencing in the next interphase.