Red 5 has developed a small molecule that represents the first-in-class theranostic agent to treat acute lymphoblastic leukemia (ALL). ALL is the most common type of childhood cancer, presenting with approximately 5,000 new cases each year in the United States. Although the cause of ALL is remains elusive, significant evidence exists for the involvement of an important enzyme designated terminal deoxynucleotidyl transferase (TdT). This unique DNA polymerase is overexpressed in ~90% of ALL patients. In addition, higher TdT levels correlate with poor prognoses, especially with respect to drug resistance. TdT is an exceptional therapeutic target as it is the only DNA polymerase that replicates single-stranded DNA rather than duplex DNA. This unique activity contrasts all other cellular DNA polymerases, and this distinction makes TdT an attractive target to develop selective nucleoside analogs to treat ALL.