Required for genome-wide de novo methylation and is essential for the establishment of DNA methylation patterns during development. DNA methylation is coordinated with methylation of histones. May preferentially methylates nucleosomal DNA within the nucleosome core region. May function as transcriptional co-repressor by associating with CBX4 and independently of DNA methylation. Seems to be involved in gene silencing (By similarity). In association with DNMT1 and via the recruitment of CTCFL/BORIS, involved in activation of BAG1 gene expression by modulating dimethylation of promoter histone H3 at H3K4 and H3K9. Isoforms 4 and 5 are probably not functional due to the deletion of two conserved methyltransferase motifs. Functions as a transcriptional corepressor by associating with ZHX1. Required for DUX4 silencing in somatic cells (PubMed:27153398). {ECO:0000250, ECO:0000269|PubMed:16357870, ECO:0000269|PubMed:17303076, ECO:0000269|PubMed:18413740, ECO:0000269|PubMed:18567530, ECO:0000269|PubMed:27153398}. This is the function of DNMT3B (DNA methyltransferase 3 beta, ENSG00000088305).