Final answer:
In epigenetic reprogramming, DNA methylation must be removed from key genes to reprogram mature cells to pluripotent cells.
Step-by-step explanation:
To reprogram mature cells to pluripotent cells in epigenetic reprogramming, what must be removed from key genes that are normally turned off in adult cells but are active in embryonic cells is DNA methylation. DNA methylation commonly correlates with silenced genes in adult cells. In the context of epigenetic regulation, the addition of methyl groups to cytosine DNA residues in CpG islands often leads to such silencing. Conversely, removing these methyl groups can reactivate the genes, allowing them to be transcribed once again. Histone modifications are also involved in epigenetic regulation, such as acetylation and deacetylation altering chromatin conformation; however, the specific deactivation in adult cells concerning pluripotency is attributed to DNA methylation. In cancer research, this understanding has led to therapeutic interventions that reverse silencing in genes by targeting the reversible nature of DNA methylation.