DNA methylation plays a central role in the epigenetic regulation of gene 
expression in vertebrates. Genetic and biochemical data indicated that DNA 
methyltransferase 1 (Dnmt1) is indispensable for the maintenance of DNA 
methylation patterns in mice, but targeting of the DNMT1 locus in human HCT116 
tumor cells had only minor effects on genomic methylation and cell viability. In 
this study, we identified an alternative splicing in these cells that bypasses 
the disrupting selective marker and results in a catalytically active DNMT1 
protein lacking the proliferating cell nuclear antigen-binding domain required 
for association with the replication machinery. Using a mechanism-based trapping 
assay, we show that this truncated DNMT1 protein displays only twofold reduced 
postreplicative DNA methylation maintenance activity in vivo. RNA 
interference-mediated knockdown of this truncated DNMT1 results in global 
genomic hypomethylation and cell death. These results indicate that DNMT1 is 
essential in mouse and human cells, but direct coupling of the replication of 
genetic and epigenetic information is not strictly required.