Catalytically inactive regulatory factor of DNA methyltransferases that can either promote or inhibit DNA methylation depending on the context (PubMed:11719692, PubMed:15318244, PubMed:15671018, PubMed:24074865). Essential for the function of DNMT3A and DNMT3B: activates DNMT3A and DNMT3B by binding to their catalytic domain (PubMed:15671018). Acts by accelerating the binding of DNA and S-adenosyl-L-methionine (AdoMet) to the methyltransferases and dissociates from the complex after DNA binding to the methyltransferases (PubMed:15671018). Recognizes unmethylated histone H3 lysine 4 (H3K4me0) and induces de novo DNA methylation by recruitment or activation of DNMT3 (By similarity). Plays a key role in embryonic stem cells and germ cells (PubMed:11719692, PubMed:15318244, PubMed:24074865). In germ cells, required for the methylation of imprinted loci together with DNMT3A (PubMed:11719692). In male germ cells, specifically required to methylate retrotransposons, preventing their mobilization (PubMed:15318244). Plays a key role in embryonic stem cells (ESCs) by acting both as an positive and negative regulator of DNA methylation (PubMed:24074865). While it promotes DNA methylation of housekeeping genes together with DNMT3A and DNMT3B, it also acts as an inhibitor of DNA methylation at the promoter of bivalent genes (PubMed:24074865). Interacts with the EZH2 component of the PRC2/EED-EZH2 complex, preventing interaction of DNMT3A and DNMT3B with the PRC2/EED-EZH2 complex, leading to maintain low methylation levels at the promoters of bivalent genes (PubMed:24074865). Promotes differentiation of ESCs into primordial germ cells by inhibiting DNA methylation at the promoter of RHOX5, thereby activating its expression (PubMed:24074865). Homodimer (By similarity). Heterotetramer composed of 1 DNMT3A homodimer and 2 DNMT3L subunits (DNMT3L-DNMT3A-DNMT3A-DNMT3L) (By similarity). Interacts with histone H3 (via N-terminus); interaction is strongly inhibited by methylation at lysine 4 (H3K4me) (By similarity). Interacts with EZH2; the interaction is direct (PubMed:24074865). Interacts with SPOCD1 (PubMed:32674113). Expressed in testis, thymus, ovary, and heart (PubMed:11306809). In testis, first observed in non-dividing prospermatogonia after 12.5 dpc and is highest at about the time of birth; expression declines rapidly after birth and is extinguished by 6 days post partum, when most prospermatogonia have differentiated into dividing spermatogonial stem cells (PubMed:15318244). Oogenesis in female mice takes place normally, but the heterozygous offspring of homozygous mutant females die before mid-gestation due to biallelic expression of imprinted genes normally methylated and silenced on the allele of maternal origin (PubMed:11719692). Male mice are viable but sterile, with a complete absence of germ cells in adult males (PubMed:11719692). Spermatocytes show asynapsis or abnormal synapsis, and do not progress to the full pachytene stage due to demethylation of methylation of both long-terminal-repeat (LTR) and non-LTR retrotransposons (PubMed:15318244).