The characteristic, slow action of the DOT1L inhibitors could be due to a long time needed for a series of cellular events that lead to proliferation arrest, including H3K79 methylation inhibition (with the maximal effect occurring in ~4 days) followed by decreased mRNA expression of the target genes, as well as ultimately the depletion of the gene products (proteins) key to cancer proliferation. The gene discussed is DOT1L; the disease is cancer.