In 2008, a team of scientists at Washington University School of Medicine in St. Louis became the first to decode the DNA of a patient’s cancer cells and trace the disease to its genetic roots. The patient, a woman in her 50s, suffered from acute myeloid leukemia (AML), an aggressive and often deadly cancer of the blood and bone marrow. The findings garnered the research team worldwide acclaim and paved the way for more personalized approaches for the treatment of cancer based on the clusters of mutations in patients’ tumors. One of the most unusual mutations discovered in that patient’s AML cells was in the gene DNMT3A. The gene had never been linked to cancer, so the significance of the gene’s mutations was unknown. But it was a fascinating candidate gene to consider. DNMT3A was known to encode an enzyme that can methylate DNA at very specific places in the genome, a process that can change patterns of gene expression and that was known to be very important for normal development.