This results in the restoration of the guanine within the genomic DNA. The alkyl group transfer leads to irreversible inactivation of the transferase and targets it for ubiquitylation and proteasome-mediated degradation. This so-called ‘suicide reaction’ implies that the cells’ capacity for repairing O6-guanine lesions depends on the pre-existing MGMT levels in the cell, or the rate at which cells can resynthesize MGMT.