Top: Proposed bivalent, epigenetic mechanism for regulating the brain's response to ischemia. Increased PcG protein abundance suppresses transcription. Increased TrxG protein abundance activates transcription. Bottom: In tolerance (TOL), PcG proteins increase. In injurious ischemia (INJ), TrxG proteins increase. The balance between PcG and TrxG activity may determine the response to ischemia. Transcriptional suppression and tolerance may be induced increasing PcG proteins, decreasing TrxG proteins, or a combination thereof.