One inaccurate process for repairing psoralen crosslinks appears to employ a DNA polymerase to fill in the gap formed in the strand with the two incisions. This process is inaccurate because the complementary un-incised strand still retains a portion of the crosslink and thus cannot serve as an adequate template for accurate repair synthesis. Inaccurate repair synthesis can cause mutation. Psoralen monoadducts in the template DNA strand may also cause inaccurate replication bypass (translesion synthesis) that can lead to mutation. In phage T4, the increase in mutation observed after PUVA treatment was found to reflect translesion synthesis by wild-type DNA polymerase, likely due to imperfect proof reading capability.