 DNMT1 is responsible for maintenance DNA methylation while DNMT3A and DNMT3B carry out both maintenance – correcting the errors of DNMT1 – and de novo DNA methylation.After DNMT1 knockout in human cancer cells, these cells were found to retain their inherited methylation pattern, which suggests maintenance activity by the expressed DNMT3s.DNMT3s show equal affinity for unmethylated and hemimethylated DNA substrates while DNMT1 has a 10-40 fold preference for hemimethylated DNA.