Final answer:
Dnmt3a and Dnmt3b are important for de novo DNA methylation, an epigenetic modification that leads to gene silencing.
Step-by-step explanation:
Dnmt3a and Dnmt3b are important for de novo DNA methylation. DNA methylation is an epigenetic modification that involves adding a methyl group to the DNA molecule, leading to gene silencing. Dnmt3a and Dnmt3b are DNA methyltransferases that play a key role in establishing new methylation patterns during development.