The post-translational modification of histones has been implicated in the regulation of cellular lifespan. Previously, we reported that cellular aging is associated with increased ubiquitylation of histone H2B and methylation of histone H3 at lysines 4 and 79 in yeast telomeric heterochromatin. Here, we show the antagonistic role of Set2 methyltransferase, which is specific for histone H3 at lysine 36, in regulating telomeric silencing and cellular lifespan. We observed that an intermediate state of chromatin, namely, unstable ON telomeres, exists when a gene is switched on near telomeres. This unstable state of chromatin is temporally maintained in a transcription-dependent manner and is preferentially restored to its original heterochromatic state, namely, OFF telomeres. We found that Set2 suppresses the restoration of unstable ON telomeres to the stable OFF state and promotes cellular aging. Our results suggest that the accumulation of unstable ON telomeres maintained by Set2 is one of the features of aged cells. (C) 2014 Elsevier Inc. All rights reserved.